Sahih Muslim Book 10, Hadith Number 3891.

Chapter : Hoarding of foodstuff is forbidden.

Jabir (Allah be pleased with him) reported: My camel had grown tired as Allah’s Messenger (may peace be upon him) came to me. He goaded it and it began to jump. After that I tried to restrain its rein so that I could listen to his (Prophet’s) words, but I could not do that. Allah’s Apostle (may peace be upon him) met me and said: Sell it to me, and I sold it for five ‘uqiyas. I said: On the condition that I may use it as a ride (for going back) to Medina. He (the Holy Prophet) said: Well, you may use it as a ride up till Medina. When I came to Medina I handed over that to him and he made an addition of an uqiya (to that amount which had been agreed upon) and then presented that (camel) to me.