Musnad Abu Ya'la al-Mawsili 115:189

Muhammad ibn Mahdi narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah, from Anas: The Messenger of Allah (peace and blessings of Allah be upon him) said, "I intend to fast, so do you have anything?" He said: So I brought him a tray containing dates and a vessel containing water after Bilal had given the adhan. He said, "Look for someone who will eat." So I went out and found Zayd ibn Thabit and invited him. He said, "O Messenger of Allah, I have drunk a drink of sawiq (barley meal) and I intend to fast." The Messenger of Allah (peace and blessings of Allah be upon him) said, "And I intend to fast." Then he ate suhur with him, then he prayed two rak'ahs, then he went out, and the prayer was called.