Musannaf Ibn Abi Shayba 1:444

Hushaym, Ibn ‘Ulayyah, and Mu‘tamir narrated to us, from Ishaq ibn Suwayd al-‘Adawi, who said: Al-‘Ala’ ibn Ziyad narrated to us, saying: The Messenger of Allah (may Allah bless him and grant him peace) performed ghusl (a ritual bath) due to janabah (major ritual impurity). He then went out and saw a spot on his shoulder that the water had not reached. He took hold of his hair (forelock) and moistened it with it.