Musannaf Ibn Abi Shayba 13:469

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Abu Bishr, from Nafi’, from Ibn Umar, that he bought a she-camel for four camels at al-Rabadhah. He said to his companion: "Go and look, and if you are satisfied, then the sale is binding."