Abu Dawud Book 2, Hadith Number 715.

Narated By Abdullah ibn Abbas : AbusSahba’ said: We discussed the things that cut off the prayer according to Ibn Abbas. He said: I and a boy from Banu AbdulMuttalib came riding a donkey, and the Apostle of Allah (PBUH) was leading the people in prayer. He dismounted and I also dismounted. I left the donkey in front of the row (of the worshippers). He (the Prophet) did not pay attention to that. Then two girls from Banu AbdulMuttalib came and joined the row in the middle, but he paid no attention to that.