Hadith 517470
Chapter 54: General Subjects - كتاب الاستئذان
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 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.' "
Reference Muwatta Malik 517470
USC-MSA web (English) reference : Book 54, Hadith 2