Musnad Abu Ya'la al-Mawsili 2:30

Abu Bakr ibn Abi Shaybah narrated to us, Ibn Idris narrated to us, from ‘Asim ibn Kulayb, from his father, who said: I met ‘Umar while he was at the Mawsim (pilgrimage site), and I called out to him from behind the tent, "Indeed, I am so-and-so, son of so-and-so, the Jurmi, and a nephew of ours has a brother who is a captive among the tribe of so-and-so. We have offered him the obligatory payment (fariḍah) of the Messenger of Allah, may Allah bless him and grant him peace, but he refused." He said: So ‘Umar lifted the side of the tent and said, "Do you recognize your companion?" I said, "Yes, that is him." He said, "Go with him until he carries out for you the judgment of the Messenger of Allah, may Allah bless him and grant him peace." He said: And we used to say that the judgment was four camels.