Maliks Muwatta Book 054, Hadith Number 002.

Section : Asking Permission to Enter.

Malik related to me from a reliable source of his from Bukayr ibn Abdullah ibn al-Ashajj from Basr ibn Said from Abu Said al-Khudri that Abu Musa al-Ashari said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'One asks permission three times. If you are given permission, then enter. If not, go away.'"