Musannaf Ibn Abi Shayba 3:1521

Abu Bakr narrated to us, he said: Abdullah ibn Idris narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about a drop of urine that touched a leather sock. One of them said: "He repeats [the prayer]," and the other said: "He does not repeat [the prayer]."