Muwatta Malik 147

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 147

In-book

Book 3, Hadith 2