من لا يحضره الفقيه, volume 1, page 288

Hadith.891 - Harith ibn al-Mughira al-Nadri narrated from Imam Jafar ibn Muhammad Al-Sadiq (as) that He said: "Whoever hears the mu'adhdhin (caller to prayer) say, 'I bear witness that there is no god but Allah (swt), and I bear witness that Muhammad is the Messenger of Allah (sw),' and then, affirming and seeking reward, responds with 'And I also bear witness that there is no god but Allah (swt), and I bear witness that Muhammad is the Messenger of Allah (sw). I suffice myself with these testimonies against anyone who denies or rejects, and I support with them anyone who affirms and bears witness,' will receive a reward equivalent to the number of those who deny and reject and the number of those who affirm and bear witness."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.892 - Imam Muhammad ibn Ali Al-Baqir (as) said to Muhammad ibn Muslim: "O’ Muhammad ibn Muslim, never neglect the remembrance of Allah (swt) in any situation. Even if you hear the caller calling the adhan while you are in a state of relieving yourself, remember Allah (swt), the Mighty and Majestic, and say what the mu'adhdhin says."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.893 - Zaid al-Shahham asked Imam Jafar ibn Muhammad Al-Sadiq (as) about a man who forgot the adhan (call to prayer) and iqamah (second call to prayer) and began the prayer. The Imam said: "If he remembers before reciting (the Qur'an in prayer), he should send blessings upon the Prophet and his family and then recite the iqamah. But if he has already started reciting, then he should complete his prayer”.

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.