Muwatta Malik, Book 3, Hadith 2 (USC-MSA) ⓘ
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."
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 " .
also referenced under: Book 3, Hadith 148 (In-Book)