Musnad Abu Ya'la al-Mawsili 154:114

Narrated to us Salt ibn Mas'ud al-Jahdari, narrated to us 'Ikrimah ibn Khalid ibn Salamah al-Makhzumi, he said: I heard my father say: I heard Ibn 'Umar say: The Messenger of Allah (peace be upon him) said: "Do not strike the slaves, for you do not know what you might encounter [of their state or what they might be doing]."