Musnad Ahmad 176:2

Ibn Numayr narrated to us, Yahya narrated to us, from Bashir ibn Yasar, from Suwayd ibn al-Nu’man, who said: "We went out with the Messenger of Allah, may Allah bless him and grant him peace, in the year of Khaybar, until when we were at al-Sahba’, and he prayed the Asr prayer, he called for food. Nothing was brought to him except *sawiq* (barley meal), so they ate and drank from it. Then he stood for the Maghrib prayer, and he rinsed his mouth, and we rinsed our mouths with him, and he did not touch water [for ablution]."