Sa'd bin Abu Waqqas (May Allah be pleased with him) reported

The Prophet (ﷺ) said, "He who says after the Adhan: 'Ash-hadu an la ilaha illallah Wah-dahu la sharika Lahu; wa ash-hadu anna Muhammad an 'abduhu wa Rasuluhu, radhitu Billahi Rabban, wa bi Muhammadin Rasulan, wa bil Islami Dinan [I testify that there is no true god except Allah Alone; He has no partners and that Muhammad (ﷺ) is His slave and Messenger; I am content with Allah as my Rubb, with Muhammad as my Messenger and with Islam as my Deen],' his sins will be forgiven."

[Muslim].