I found it in a book.
Gautama Buddha himself enlisted the various
duties of the students in the Vinaya texts: "Let the bhikshu give to the preceptor the teeth-cleaner and water to rinse his mouth with, then let him prepare a seat. If there is rice milk let him rinse the jug and offer rice milk to the
5 preceptor. When he had drunk it let him give water .
The footnote says Mahavagga. I, 25;8
How do I find it? Help me please.