It was narrated from Ibn 'Abbas:
"The Prophet bathed to cleanse himself from sexual impurity, then he saw a spot that the water did not reach." Then he motioned with the hair hanging over his shoulders and squeezed (the water from it) over that spot." (Da'if)In his narration, Ishaq said: "So he wrung his hair over it."
English reference: Vol. 1, Book 1, Hadith 663
Arabic reference: Book 1, Hadith 708