Saad bin Abu Waqqas reported,

The Messenger of Allah ﷺ said, "Whoever says when he hears the caller to prayer, 'I too bear witness that there is no god but Allah alone, with no partner, and that Muhammad is His servant and messenger. I am content with Allah as my Lord, with Muhammad as my Messenger, and with Islam as my religion,' his sins will be forgiven."

Tirmidhi 210

Sahih

حدثنا قتيبة حدثنا الليث عن الحكيم بن عبد الله بن قيس عن عامر بن سعد

عن سعد بن أبي وقاص

عن رسول الله ﷺ قال من قال حين يسمع المؤذن وأنا أشهد أن لا إله إلا الله وحده لا شريك له وأن محمدا عبده ورسوله رضيت بالله ربا وبمحمد رسولا وبالإسلام دينا غفر له ذنبه

الترمذي 210صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami Tirmidhi
Book
Prayer
Chapter
What a man should say as supplication when the muezzin calls the adhan
Numbering
Darussalam 210
Tirmidhi 210