Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Whoever believes in Allah Almighty and the Last Day should honour his guest." He said this three times. He (the Companion) asked: "O Messenger of Allah, what is meant by honouring the guest?" The Prophet (peace and blessings be upon him) said: "Three days (of hospitality); if the guest stays after that, it is charity upon him."
Imam Ahmad recorded it at length like this, and also in abridged form with various chains; Imam Abu Ya'la and Imam al-Bazzar also recorded it. The narrators of one of Imam Ahmad's chains are the narrators of the Sahih.