The Messenger of Allah ﷺ said, "When you hear the call to prayer, repeat what the muezzin says."
حدثنا عبد الله بن مسلمة القعنبي عن مالك عن ابن شهاب عن عطاء بن يزيد الليثي
أن رسول الله ﷺ قال إذا سمعتم النداء فقولوا مثل ما يقول المؤذن
Bukhari reported it in the Book of Adhan, chapter on what one should say when hearing the caller, hadith no. 611, and Muslim in the Book of Prayer, chapter on the recommendation of repeating the words of the muezzin for one who hears them, hadith no. 383, from the hadith of Malik through this chain. It is also found in Muwatta (narrated by Yahya): 1/67, and (narrated by) Qanabi, pp. 84–85.