Musnad Ahmad 25:342

'Ali ibn 'Asim narrated to us, Abu 'Ali al-Rahbi narrated to us, from 'Ikrimah, Ibn 'Abbas informed us, he said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed ghusl (a ritual bath) due to janabah (major ritual impurity). When he came out, he saw a spot on his left shoulder that the water had not reached. So he took some of his hair and moistened it, then he proceeded to the prayer."