Hadith 501480
Chapter 3: Prayer - كتاب الصلاة
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَطَاءِ بْنِ يَزِيدَ اللَّيْثِيِّ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا سَمِعْتُمُ النِّدَاءَ فَقُولُوا مِثْلَ مَا يَقُولُ الْمُؤَذِّنُ " .
Yahya related to me from Malik from Ibn Shihab from Ata ibn Yazid al-Laythi from Abu Said al-Khudri that the Messenger of Allah, may Allah bless him and grant him peace, said, "When you hear the adhan, repeat what the muadhdhin says."
Reference Muwatta Malik 501480
USC-MSA web (English) reference : Book 3, Hadith 2