Sahih Muslim 383
Sahih Muslim · Book 4: The Book of Prayers · It Is Recommended For The One Who Hears The Mu'adhdhin To Repeat His Words, Then To Send Salat Upon The Prophet (saws) And Ask Allah To Grant Him Al-Wasilah
Abu Sa'id al-Khudri reported:
حَدَّثَنِي يَحْيَى بْنُ يَحْيَى ، قَالَ قَرَأْتُ عَلَى 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 " .
When you hear the call (to prayer), repeat what the Mu'adhdhin pronounces.