Sahih Bukhari Volume 8, Book 73, Hadith Number 156.

Narated By Abu Shuraih Al-Ka’bi : Allah’s Apostle said, Whoever believes in Allah and the Last Day, should serve his guest generously. The guest’s reward is: To provide him with a superior type of food for a night and a day and a guest is to be entertained with food for three days, and whatever is offered beyond that, is regarded as something given in charity. And it is not lawful for a guest to stay with his host for such a long period so as to put him in a critical position.”