Sahih Bukhari Volume 7, Book 65, Hadith Number 302.

Narrated By Suwaid bin An-Nu’man : That while they were with the Prophet at As-Sahba’ which was at a distance of one day’s journey from Khaibar the prayer became due, and the Prophet asked the people for food but there was nothing with the people except Sawiq. He ate of it and we ate along with him, and then he asked for water and rinsed his mouth (with it), and then offered the (Maghrib) prayer and we too offered the prayer but the Prophet did not perform ablution (again after eating the Sawiq.).