General Behavior (Kitab Al-Adab)
Dawud :: Book 41 : Hadith 5158
Narrated Rib'i:
A man of Banu Amir told that he asked the Prophet (peace_be_upon_him) for permission (to enter the house) when he was in the house, saying: May I enter ? The Prophet (peace_be_upon_him) said to his servant: Go out to this (man) and teach him how to ask permission to enter the house, and say to him: "Say : Peace be upon you. May I enter?" The man heard it and said: Peace be upon you! May I enter ? The Prophet (peace_be_upon_him) permitted him and he entered.