How do we decide what was most likely spoken by the historical buddha and what was not? Mind you, let us not make slippery slope arguments.
I have heard that at one time the Blessed One was staying at Vesali, in the Peaked Roof Hall in the Great Forest.
Then Mahapajapati Gotami went to the Blessed One and, on arrival, having bowed down to him, stood to one side. As she was standing there she said to him: "It would be good, lord, if the Blessed One would teach me the Dhamma in brief such that, having heard the Dhamma from the Blessed One, I might dwell alone, secluded, heedful, ardent, & resolute."
"Gotami, the qualities of which you may know, 'These qualities lead to passion, not to dispassion; to being fettered, not to being unfettered; to accumulating, not to shedding; to self-aggrandizement, not to modesty; to discontent, not to contentment; to entanglement, not to seclusion; to laziness, not to aroused persistence; to being burdensome, not to being unburdensome': You may categorically hold, 'This is not the Dhamma, this is not the Vinaya, this is not the Teacher's instruction.'
"As for the qualities of which you may know, 'These qualities lead to dispassion, not to passion; to being unfettered, not to being fettered; to shedding, not to accumulating; to modesty, not to self-aggrandizement; to contentment, not to discontent; to seclusion, not to entanglement; to aroused persistence, not to laziness; to being unburdensome, not to being burdensome': You may categorically hold, 'This is the Dhamma, this is the Vinaya, this is the Teacher's instruction.'"
That is what the Blessed One said. Gratified, Mahapajapati Gotami delighted at his words.
And what is right speech? Abstaining from lying, from divisive speech, from abusive speech, & from idle chatter: This is called right speech.