Authentic Supplication Number 212.

A man from Bani `Amir related that he asked permission [to enter] upon the Prophet  when he was in a house, so he said: Penetrate I? So the Prophet  told his servant: Go out to this [person] and teach him how to ask permission to enter, thus, telll him: say 'As-Salamu `alaykum 'a'adhkhulu? (shall I enter?).

The man, thereupon, heard this, and thus said: 'As-Salamu `alaykum 'a'adhkhulu?. Thus the Prophet  gave him permission [to enter] and he entered.

'Al-`Adawee said it was sound, and that it was reported by 'Abu Dawood, volume 5 page 369; and Al-Bukhari, #1084 in 'Al-'Adab 'Al-Mufrad.