Musannaf Ibn Abi Shayba 38:185

Shababah narrated to us, from al-Hudhayl ibn Bilal, who said: I was with Muhammad ibn Sirin when a man came to him and said: "I have a slave whom I wish to sell. I have been offered six hundred dirhams for him, and the Khawarij have offered me eight hundred. Should I sell him to them?" He said: "Would you sell him to a Jew or a Christian?" He said: "No." He said: "Then do not sell him to them."