من لا يحضره الفقيه, volume 1, page 395

Hadith.1170 - The Commander of the Faithful Imam Ali ibn Abi Talib (as) said: "There is no problem if a young boy (who has not yet reached puberty) gives the adhan (call to prayer), but he should not lead the prayer until he has reached puberty. If he leads, his own prayer is valid, but the prayer of those who follow him is invalid."

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

Hadith.1171 - Ammar al-Sabati asked Imam Jafar ibn Muhammad Al-Sadiq (as) about a man who arrives at the mosque just as the Imam is concluding the prayer with the taslim (final salutation). Imam (as) replied: "He should give the adhan (call to prayer), make the iqamah (call to commence the prayer), and then begin his prayer."

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

Hadith.1172 - A question was asked about a man who enters the mosque while the congregation is already in prayer, and the Imam is ahead of him by one rak'ah. He pronounces the takbir, and then the Imam becomes unable to continue (due to illness or another reason) and takes the man’s hand (indicating him to lead), and he happens to be the nearest person to the Imam. Imam (as) said: "He should lead them in completing the prayer, then sit until they finish their tashahhud (final testimony). He should gesture to the right and left with his hand as a sign of completing the prayer (taslim) or wait until the prayer is completed by the congregation, after which he can complete what he missed of his own prayer."

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

Hadith.1173 - Muhammad ibn Sahl narrated from his father who said: “I asked Imam Al-Ridha (as) about a person who bows (in ruku') with an Imam of a congregation who is followed, and then raises his head before the Imam.” Imam (as) replied: "He should repeat his bowing (ruku') with him."

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