Riyad as-Salihin 872 — The Book of Greetings

Rib'i bin Hirash Narrated from:

A man of Banu 'Amir tribe has told us that he had asked the Prophet (ﷺ) for permission to enter when he was at home. He said: "May I enter?" the Messenger of Allah (ﷺ) 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 (ﷺ) then granted him permission and he entered. [Abu Dawud].