Malik related to me from a reliable source of his from Bukayr ibnAbdullah ibn al-Ashajj from Basr ibn Said from Abu Said al-Khudri thatAbu Musa al-Ashari said, "The Messenger of Allah, may Allah bless himand grant him peace, said, 'One asks permission three times. If youare given permission, then enter. If not, go away.' "
USC-MSA web (English) reference: Book 54, Hadith 2
Arabic reference: Book 54, Hadith 1767