Musannaf Ibn Abi Shayba 13:562

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, from Sa’id ibn al-Musayyib, that the people said: "We wish we had seen a transaction between Abd al-Rahman ibn Awf and Uthman, so that we could see which of them is more diligent in trade." So Abd al-Rahman bought horses from Uthman for forty thousand, and he stipulated to him: "If the transaction reaches them while they are alive, gathered with the shepherd, and not lost, then the sale is binding." Then he passed by something and said: "What have I done?" So he returned to him and said: "I will add six thousand to you, on the condition that if the messenger reaches them and they are alive, then it is upon me." The messenger reached them, and they had died. So Abd al-Rahman was released from the liability by the second condition.