Musnad Ahmad 1156:32

Yahya ibn Sa'id narrated to us, from Ja'far ibn Muhammad, who said: My father narrated to me, from 'Ali ibn al-Husayn, from Zaynab bint Umm Salamah, from Umm Salamah: "The Messenger of Allah (peace and blessings of Allah be upon him) ate a shoulder (of a sheep), then Bilal came to him, so he went out to the prayer and did not touch water."