From Sahih Bukhari (Hadith 4412):
Narrated `Abdullah bin `Abbas:That he came riding a donkey when Allah's Apostle was standing at Mina during Hajjat-ul-Wada`, leading the people in prayer. The donkey passed in front of a part of the row (of the people offering the prayer). Then he dismounted from it and took his position in the row with the people
Reference: Bukhari 4412