Musannaf Ibn Abi Shayba 1:1494

Abu Bakr narrated to us, he said: Isma'il bin 'Ulayyah narrated to us, from Abu Harun al-Ghanawi, from Abu Mijlaz, he said: Ibn 'Umar said: "If one of you performs ghusl (full body ablution) for janabah (major ritual impurity) and urinates before he finishes his washing, then let him pour water over his head."