Abu al-Zubayr narrates from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) came while my camel had become exhausted. He (peace and blessings be upon him) urged it on and it began to run; after that I kept trying to control its reins, but I could not. the Prophet (peace and blessings be upon him) caught up with me and said: "Sell it to me." I sold it to him for five uqiyahs and stipulated riding it as far as Madinah. He (peace and blessings be upon him) said: "You may ride it as far as Madinah." When I came to Madinah, I brought the camel to him (peace and blessings be upon him); he (peace and blessings be upon him) added one uqiyah and then gave it to me as a gift.
Note: Some of the wordings indicate that a condition was made in the sale, and some that a kindness was shown after the sale.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10843