Musnad Ahmad 251:1

Yahya ibn Sa'id narrated to us, from Yahya ibn Sa'id al-Ansari, who said: Bashir ibn Yasar narrated to me, from Suwayd ibn al-Nu'man: "That the Messenger of Allah (peace and blessings of Allah be upon him) stopped at al-Sahba' in the year of Khaybar. When he had prayed the Asr prayer, he called for food, but nothing was brought to him except sawiq (parched barley meal). He said: So we ate from it. Then, when it was time for the Maghrib prayer, he rinsed his mouth, and we rinsed our mouths with him."