This is one sutta where it is mentioned:
8. "Bhikkhus. Ariyan disciples in this Religion reflect thus:
"'All arahants, for as long as life lasts, have given up lying on large or high beds. They are content with low beds or bedding made of grass.'
"All of you have given up lying on large or high beds. You are content with low beds or beds made of grass. For all of this day and night, in this manner, you will be known as having followed the arahants, and the Uposatha will have been observed by you. This is the eighth factor of the Uposatha."
"Bhikkhus. The Uposatha is comprised of these eight factors which the Ariyan disciple observes, and it is of great and glorious fruit and benefit."
Thus the Blessed One spoke on the Uposatha. The bhikkhus were delighted and rejoiced at his words.http://www.accesstoinsight.org/lib/auth ... satha.html