Daraqutni

Shaykh Abu Wasi narrates: We were participating in a military expedition, when a man came along with his horse. One of us said: "Will you sell this horse to him in exchange for this slave?" He said: "Yes!" Then he sold it to him. He stayed with us that night, and when morning came, he went towards his horse. Our companion said to him: "What do you have to do with this horse? Did you not sell it to me?" The man replied: "I do not want this transaction." Our companion said: "Now you have nothing to do with it, you have sold it to me." The people present there said to both of them: "Here is Abu Bardah, a companion of the Prophet (peace be upon him)." Both of them went to him, and Sayyiduna Abu Barzah said to them: "Are you both satisfied with the decision of the Prophet (peace be upon him) in this matter?" Both replied: "Yes!" Sayyiduna Abu Hurairah (may Allah be pleased with him) reported that the Prophet (peace be upon him) said: "The two parties involved in a transaction have the right to cancel it as long as they have not separated from each other." And as for you two, I think you have separated from each other.