Hadith 872

Rib'i bin Hirash (May Allah be pleased with him) reported: A man of Banu 'Amir tribe has told us that he had asked the Prophet (PBUH) for permission to enter when he was at home. He said: "May I enter?" the Messenger of Allah (PBUH) said to the servant, "Go out and instruct him about the manner of seeking permission. Tell him to say: As-Salamu 'Alaikum (may peace be upon you). May I come in?" The man heard this and said: "As-Salamu 'Alaikum (may peace be upon you). May I come in?" The Prophet (PBUH) then granted him permission and he entered.

[Abu Dawud].