Musannaf Ibn Abi Shayba 9:1597

Waki’ narrated to us, from Aflah, from al-Qasim, who said: He was asked about: "And those who guard their private parts, except from their wives or those their right hands possess, for indeed, they are not to be blamed. But whoever seeks beyond that, then those are the transgressors." He said: Whoever seeks beyond that is a transgressor.