Sahih Muslim Book 19, Hadith Number 4464.

It has been narrated on the authority of Abu Ishaq that ‘Abdullah b. Yazid went (out of the city) with people for offering “Istisqa”‘ prayer (for rainfall). He offered two rak’ahs. Then he prayed for rain. That day I met Zaid b. Arqam. There was only one man between me and him (at that time). I asked him: How many military expeditions did the Messenger of Allah (may peace be upon him) undertake? He said: Nineteen expeditions. I asked him: On how many expeditions did you accompany him? He said: On seventeen expeditions. I asked: Which was the first expedition he led? He answered: Dhat-ul-Usair or ‘Ushair.