(7) باب اسْتِحْبَابِ الْقَوْلِ مِثْلَ قَوْلِ الْمُؤَذِّنِ لِمَنْ سَمِعَهُ ثُمَّ يُصَلِّي عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ثُمَّ يَسْأَلُ اللَّهَ لَهُ الْوَسِيلَةَ
Abu Sa'id al-Khudri reported:
When you hear the call (to prayer), repeat what the Mu'adhdhin pronounces.
Reference: Sahih Muslim 383
In-book reference: Book 4, Hadith 12
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 746