What is the reward for reciting specific supplications upon hearing the call to prayer (Adhan)?

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If one recites upon hearing the Adhan: 'I bear witness that there is no god but Allah alone, He has no partner, and that Muhammad is His Messenger. I am content with Allah as my Lord, with Islam as my religion, and with Muhammad as my Messenger,' their sins shall be forgiven. This is supported by the narration of Sa'd ibn Abi Waqqas, as recorded by Muslim.