من لا يحضره الفقيه, volume 1, page 352

Hadith.1027 - Muhammad ibn Muslim narrates from Imam Jafar ibn Muhammad Al-Sadiq (as): "If a man is uncertain after he has prayed and does not know whether he prayed three or four rak'ahs, and when he finishes, he is certain that he has completed it, he does not need to repeat the prayer. At the time of his departure (after the prayer), his certainty is closer to the truth than it was afterward."

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

Hadith.1028 – In the Nawaadir of Ibrahim ibn Hashim, it is narrated that Imam Jafar ibn Muhammad Al-Sadiq (as) was asked about an Imam leading a prayer with four or five people, and two of them testify that they prayed three rak'ahs, while three testify that they prayed four rak'ahs. Some say, "Stand up," and others say, "Sit down," and the Imam is unsure, either leaning towards one or being uncertain. What should be done? Imam (as) said: "There is no fault on the Imam if those behind him agree that he made a mistake. There is no fault on those behind the Imam if the Imam has not erred. There is no confusion in the Maghrib prayer, nor in Fajr, nor in the first two rak'ahs of any prayer. If there is disagreement among those behind the Imam, then both the Imam and the congregation should take precaution and repeat the prayer, acting decisively."

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