Sayyiduna al-Tilb (may Allah be pleased with him) narrates: I heard the Prophet (peace and blessings be upon him) say: "Hospitality for three days is a binding right; whatever is after that is charity."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat, and its chain contains a narrator whom I do not know.