Musnad Ahmad 8:126

Yunus ibn Muhammad narrated to us, Layth narrated to us, from al-Hakam ibn Abdullah ibn Qays, from Amir ibn Sa’d ibn Abi Waqqas, from his father, Sa’d: That the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever says when he hears the caller to prayer: 'And I bear witness that there is no god but Allah, alone, with no partner, and that Muhammad is His servant and His Messenger; I am pleased with Allah as Lord, with Muhammad as a Messenger, and with Islam as a religion,' his sin will be forgiven." Qutaybah narrated it to us, and he said: Al-Layth narrated it to us, from al-Hakim ibn Abdullah ibn Qays.