"Sahih Bukhari, Volume 9, Book 91, Number 368: Narrated '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. ... (cf. Bukhari 3.648, 6.435, 7.119)"