Sahih Bukhari Volume 003, Book 047, Hadith Number 781.
Narated By Ibn 'Umar : That he was in the company of the Prophet on a journey, riding a troublesome camel belonging to 'Umar. The camel used to go ahead of the Prophet, so Ibn 'Umar's father would say, "O 'Abdullah! No one should go ahead of the Prophet." The Prophet said to him, "Sell it to me." 'Umar said to the Prophet "It is for you." So, he bought it and said, "O 'Abdullah! It is for you, and you can do with it what you like."