Musnad Abu Ya'la al-Mawsili 147:62

Ubayd Allah ibn Umar al-Qawariri narrated to us, Fudayl ibn Iyad narrated to us, from Muslim al-Barrad, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) would accept the invitation of a slave, visit the sick, and ride a donkey.