Musnad Abu Ya'la al-Mawsili 129:22

Ibn Abi Samina al-Basri narrated to us, Mu'tamir ibn Sulayman narrated to us, from Humayd al-Tawil, from Anas: That the delegation of Thaqif said, "O Messenger of Allah, our land is a cold land, so what is sufficient for us regarding the bath for major ritual impurity (janaba)?" He said, "As for me, I pour three [handfuls] over my head."