The story about the monk reborn as a flea is from the Commentary to the Dhammapada..
Ayasava malan samutthitam
tatutthaya tameva khadati
evam atidhonacarinam1
sani kammani nayanti duggatim.
Verse 240: Just as rust is formed from iron, and corrodes the iron from which it is formed, so also, his own deeds lead the transgressor to a lower plane of existence (duggati).
1. atidhonacarinam - transgressor: i.e., one who transgresses or indulges too much in the use of 'dhona', the four requisites of a bhikkhu.
The Story of Thera Tissa
While residing at the Jetavana monastery, the Buddha uttered Verse (240) of this book, with reference to Thera Tissa.
Once there was a thera named Tissa in Savatthi. One day, he received a set of fine robes and was very pleased. He intended to wear those robes the next day. But that very night he died and because he was attached to the fine set of robes, he was reborn as a louse and lived within the folds of the robes. As there was no one to inherit his belongings, it was decided that this particular set of robes should be shared by other bhikkhus. When the bhikkhus were preparing to share out among themselves, the louse was very much agitated and cried out, "They are destroying my robes!" This cry was heard by the Buddha by means of his divine power of hearing. So he sent someone to stop the bhikkhus and instructed them to dispose of the robes only at the end of seven days. On the eighth day, the set of the robes which belonged to Thera Tissa was shared out by the bhikkhus.
Later, the Buddha was asked by the bhikkhus why he had told them to wait for seven days before sharing out the robes of Thera Tissa. To them the Buddha replied, "My sons, Tissa had his mind attached to this particular set of robes at the time of his death, and so he was reborn as a louse and stayed in the folds of the robes. When you all were preparing to share out the robes, Tissa the louse was very much in agony and was running about to and fro in the folds of the robes. If you had taken the robes at that time Tissa the louse would have felt very bitter against you and he would have to go to niraya. But now, Tissa has been reborn in the Tusita deva world, and that is why I have allowed you to take the robes. Indeed, bhikkhus, attachment is very dangerous; as rust corrodes iron from which it is formed, so also, attachment destroys one and sends one to niraya. A bhikkhu should not indulge too much in the use of the four requisites or be very much attached to them."
Then the Buddha spoke in verse as follows:
Verse 240: Just as rust is formed from iron, and corrodes the iron from which it is formed, so also, his own deeds lead the transgressor to a lower plane of existence (duggati).
Stories of the Buddha’s Former Births
Book 2 Dukanipāta
207. Assaka Jātaka
“Once with the great king Assaka,” etc.—This story the Master told whilst staying in Jetavana, about some one who was distracted by the recollection of a former wife. He asked the Brother whether he were really lovesick. The man said, Yes. “Whom are you in love with?” the Master continued. “My late wife,” was the reply. Then the Master said, “Not this once only, Brother, have you been full of desire for this woman; in olden days her love brought you to great misery.” And he told a story.
Once upon a time, there was a king Assaka reigning in Potali, which is a city of the kingdom of Kasi. His queen consort, named Ubbari, was very dear to him; she was charming, and graceful, and beautiful passing the beauty of women, though not so fair as a goddess. She died: and at her death the king was plunged in grief, and became sad and miserable. He had the body laid in a coffin, and embalmed with oil and ointment, and laid beneath the bed; and there he lay without food, weeping and wailing. In vain did his parents and kinsfolk, friends and courtiers, priests and laymen, bid him not to grieve, since all things pass away; they could not move him. As he lay in sorrow, seven days passed by.
Now the Bodhisatta was at that time an ascetic, who had gained the Five Supernatural Faculties and the Eight Attainments; he dwelt at the foot of Himalaya. He was possessed of perfect supernatural insight, and as he looked round India with his heavenly vision, he saw this king lamenting, and straightway resolved to help him. By his miraculous power he rose in the air, and alighted in the king’s park, and sat down on the ceremonial stone, like a golden image.
A young brahmin of the city of Potali entered the park, and seeing the Bodhisatta, he greeted him and sat down. The Bodhisatta began to talk pleasantly with him. “Is the king a just ruler?” he asked.
“Yes, Sir, the king is just.” replied the youth; “but his queen is just dead; he has laid her body in a coffin, and lies down lamenting her; and to-day is the seventh day since he began.—Why do you not free the king from this great grief? Virtuous beings like you ought to overcome the king’s sorrow.”
“I do not know the king, young man,” said the Bodhisatta; “but if he were to come and ask me, I would tell him the place where she has now come into the flesh again, and make her speak herself.”
“Then, holy Sir, stay here until I bring the king to you,” said the youth. The Bodhisatta agreed, and he hastened into the king’s presence, and told him about it. “You should visit this being with the divine insight!” he told the king.
The king was overjoyed, at the thought of seeing Ubbari; and he entered his chariot and drove to the place. Greeting the Bodhisatta, he sat down on one side, and asked, “Is it true, as I am told, that you know where my queen has come into being again?”
“Yes, I do, my lord king,” replied he.
Then the king asked where it was.
The Bodhisatta replied, “O king, she was intoxicated with her beauty, and so fell into negligence and did not do fair and virtuous acts; so now she has become a little dung-worm in this very park.”
“I don’t believe it!” said the king.
“Then I will show her to you, and make her speak,” answered the Bodhisatta.
“Please make her speak!” said the king.
The Bodhisatta commanded—“Let the two that are busy rolling a lump of cow-dung, come forth before the king:” and by his power he made them do it, and they came. The Bodhisatta pointed one out to the king: “There is your queen Ubbari, O king! she has just come out of this lump, following her husband the dung-worm. Look and see.”
“What! my queen Ubbari a dung-worm? I don’t believe it!” cried the king.
I will make her speak, O king!”
“Pray make her speak, holy Sir!” said he.
The Bodhisatta by his power gave her speech. “Ubbari!” said he.
“What is it, holy Sir?” she asked, in a human voice.
“What was your name in your former character?” the Bodhisatta asked her.
“My name was Ubbari, Sir,” she replied, “the consort of king Assaka.”
“Tell me,” the Bodhisatta went on, “which do you love best now—king Assaka, or this dung-worm?”
“O Sir, that was my former birth,” said she. “Then I lived with him in this park, enjoying shape and sound, scent, savour and touch; but now that my memory is confused by rebirth, what is he? Why, now I would kill king Assaka, and would smear the feet of my husband the dung-worm with the blood flowing from his throat!” and in the midst of the king’s company, she uttered these verses in a human voice:
“Once with the great king Assaka, who was my husband dear,
Beloving and beloved, I walked about this garden here.
“But now new sorrows and new joys have made the old ones flee,
And dearer far than Assaka my Worm is now to me.”
When king Assaka heard this, he repented on the spot; and at once he caused the queen’s body to be removed and washed his head. He saluted the Bodhisatta, and went back into the city; where he married another queen, and ruled in righteousness. And the Bodhisatta, having instructed the king, and set him free from sorrow, returned again to the Himalayas.
When the Master had ended this discourse, he declared the Truths and identified the Birth—at the conclusion of the Truths, the lovesick Brother reached the Fruit of the First Path—“Your late wife was Ubbari; you, the lovesick Brother, were king Assaka; Sariputta was the young brahmin; and the anchorite was I myself.”
