Abu Ishaq narrates that Abdullah bin Yazid al-Ansari (may Allah be pleased with him) went out with the people to supplicate for rain. He led two rak'ahs, then supplicated for rain. On that day I met Zayd bin Arqam (may Allah be pleased with him); there was only one man between me and him, or he said: between me and him there was a man. I said: How many military expeditions did he (peace and blessings be upon him) undertake? He said: Nineteen expeditions. Then I said: How many expeditions did you undertake with him (peace and blessings be upon him)? He said: Seventeen. Then I said: Which was the first expedition that he (peace and blessings be upon him) fought? He said: Dhat al-'Usayrah, or he said: Dhat al-'Ushayrah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6407