Musnad Abu Ya'la al-Mawsili 109:126

We were with the Messenger of Allah (may Allah bless him and grant him peace) on a journey, and he bought a camel from me and granted me its back [to ride] until I arrived in Medina. When we arrived, I came to him with the camel and handed it over to him. He ordered that the price be given to me, and then I departed. Suddenly, the Messenger of Allah (may Allah bless him and grant him peace) caught up with me. I said, "Perhaps he has changed his mind about it." He said, "When I came to him, he handed the camel back to me and said, 'It is yours.'" I passed by a man from the Jews and informed him. He began to marvel and said, "He bought the camel from you, then handed the price to you, then gifted it back to you?" I said, "Yes."