Musannaf Ibn Abi Shayba 1:769

Hatim ibn Isma'il narrated to us, from 'Abd al-Rahman ibn Harmala, that a man from his family performed ghusl (ritual bath) from janaba (major ritual impurity) and forgot to wash his head. He said: So he ordered me to ask Sa'id ibn al-Musayyib about that. I asked him, and he said: "Let him return and wash his head." He said: So I went and poured water from the wudu' (vessel) over it until he washed his head.