Silsilat al-Ahadith al-Sahihah

The Prophet Muhammad (peace and blessings be upon him) was sitting at home when a man from the tribe of Banu Amir sought permission, saying: "May I come in?" The Prophet (peace and blessings be upon him) said (to his servant): "Go to this man and teach him the manner of seeking permission, and tell him that he should seek permission with these words: 'As-salamu alaikum, may I come in?'" The man heard all of this and (acting upon it) said: "As-salamu alaikum, may I come in?" So the Prophet (peace and blessings be upon him) granted him permission and he entered.