Musannaf Ibn Abi Shayba 1:1489

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Hisham, from Hammad, from Rib’i ibn Hirash: He said: Salman said: "If one of you itches his skin, let him not wipe it with his saliva, for saliva is not pure."