Musannaf Ibn Abi Shayba 9:1754

Sufyan ibn 'Uyaynah narrated to us, from 'Amr, from Abu Ma'bad, who said: I heard Ibn 'Abbas saying: I heard the Prophet, may Allah bless him and grant him peace, saying: "Beware, a man must not be alone with a woman unless there is a mahram with her."