The Messenger of Allah ﷺ said, "When you hear the call to prayer, repeat what the muezzin says."
حدثنا إسحق بن موسى الأنصاري حدثنا معن حدثنا مالك ح قال حدثنا قتيبة عن مالك عن الزهري عن عطاء بن يزيد الليثي
قال رسول الله ﷺ إذا سمعتم النداء فقولوا مثل ما يقول المؤذن
Sourcing, agreed upon by Bukhari and Muslim. Bukhari narrated it in the Book of Adhan, chapter on what one should say upon hearing the caller, hadith: 611, and Muslim in the Book of Prayer, chapter on the recommendation of saying what the muadhin says for one who hears him... and so on, hadith: 383, from the hadith of Malik through this chain, and it is found in Muwatta: 1/67 (Yahya).There are also narrations on this subject from Abu Rafi [Ahmad: 6/9, 391], Abu Hurairah [Ibn Majah, hadith: 718], Umm Habiba [Ibn Majah, hadith: 719], Abdullah bin Amr [Muslim, hadith: 384], Abdullah bin Rabia [Nasai, hadith: 2/19, 666], Ayesha [Abu Dawud, hadith: 526], Muaz bin Anas [Ahmad: 3/438], and Muawiya [Bukhari, hadith: 612].