Musnad Abu Ya'la al-Mawsili 155:50

Mansur ibn Abi Muzahim narrated to us, Yahya ibn Hamzah narrated to us, from al-Hakam ibn Abdullah ibn Sa'd al-Ayli, that he heard al-Qasim ibn Muhammad, from Aisha, that she heard Abu Hurayrah saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Hospitality is three [days]; whatever is beyond that is charity."