Sahih Bukhari Volume 007, Book 067, Hadith Number 400.
Narrated By Abu Qatada : I was with the Prophet (on a journey) between Mecca and Medina, and all of them, (i.e. the Prophet and his companions) were in the state of Ihram, while I was not in that state. I was riding my horse and I used to be fond of ascending mountains. So while I was doing so I noticed that the people were looking at something. I went to see what it was, and behold it was an onager. I asked my companions, "What is that?" They said, "We do not know." I said, "It is an onager.' They said, "It is what you have seen." I had left my whip, so I said to them, "Hand to me my whip." They said, "We will not help you in that (in hunting the onager)." I got down, took my whip and chased the animal (on my horse) and did not stop till I killed it. I went to them and said, "Come on, carry it!" But they said, "We will not even touch it." At last I alone carried it and brought it to them. Some of them ate of it and some refused to eat of it. I said (to them), "I will ask the Prophet about it (on your behalf)." When I met the Prophet, I told him the whole story. He said to me, "Has anything of it been left with you?" I said, "Yes." He said, "Eat, for it is a meal Allah has offered to you."