وَحَدَّثَنِي مَالِكٌ، عَنِ الثِّقَةِ، عِنْدَهُ عَنْ بُكَيْرِ بْنِ عَبْدِ اللَّهِ بْنِ الأَشَجِّ، عَنْ بُسْرِ بْنِ سَعِيدٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنْ أَبِي مُوسَى الأَشْعَرِيِّ، أَنَّهُ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ الاِسْتِئْذَانُ ثَلاَثٌ فَإِنْ أُذِنَ لَكَ فَادْخُلْ وَإِلاَّ فَارْجِعْ ‏"‏ ‏.‏

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.' " Translation Not Available

Book reference: Muwatta Imam Malik Book 54 Hadith 1767
Web reference: Muwatta Imam Malik Book 54 Hadith 2