Musannaf Ibn Abi Shayba 13:2640

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Zakariyya narrated to us, from ‘Amir, who said: ‘Umar bought a horse from a man and stipulated [a condition] upon him that if he was pleased with it, [the sale would be complete], and if not, then there would be no sale between them. ‘Umar then had a man from his side ride it, and the horse perished. They appointed Shurayh as an arbitrator between them. Shurayh said to ‘Umar: "Either accept what you have purchased, or return what you have taken." ‘Umar said to him: "You have judged with the essence of the truth." Zakariyya said: ‘Amir said: And he [‘Umar] sent him [Shurayh] to the judiciary of Kufa, and he sent Ka’b ibn Sur to the judiciary of Basra.