Musannaf Ibn Abi Shaybah 31211 — The supplication on hearing the adhan

Amir bin Sa'd said that his father, Sayyiduna Sa'd (may Allah be pleased with him), said: "Whoever, when the muezzin says 'I bear witness that there is no god but Allah,' says: 'I am pleased with Allah as Lord, with Islam as religion, and with Muhammad (peace and blessings be upon him) as Prophet,' his sins are forgiven." Then a man said to him: "O Sa'd, are his past and future sins forgiven?" Sayyiduna Sa'd (may Allah be pleased with him) said: "No, this much only is what I heard the Messenger of Allah (peace and blessings be upon him) say."