Sahih Bukhari Volume 9, Book 91, Hadith Number 368.

Narrated By ‘Umar : I came and behold, Allah’s Apostle was staying on a Mashroba (attic room) and a black slave of Allah’s Apostle was at the top if its stairs. I said to him, “(Tell the Prophet) that here is ‘Umar bin Al-Khattab (asking for permission to enter).” Then he admitted me.