من لا يحضره الفقيه, volume 1, page 350

Hadith.1018 - Al-Fudail ibn Yasar asked Imam Jafar ibn Muhammad Al-Sadiq (as) about forgetfulness in prayer. Imam said: "Whoever remembers his forgetfulness and corrects it, there is no need for him to perform the two prostrations of forgetfulness (Sajdatay al-Sahw). However, forgetfulness applies to the one who does not know whether he has added to or missed part of his prayer."

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

Hadith.1019 - Al-Halabi narrated from him (the Prophet (as)) that He said: “If you are unsure whether you prayed four or five rak'ahs, or whether you added or omitted something, then perform the tashahhud (testification of faith), say salaam (greeting of peace), and prostrate two prostrations of forgetfulness without bowing or reciting (in them), and you will make a brief tashahhud in both of them.”

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

Hadith.1020 - Muhammad ibn Muslim narrated that Imam Muhammad ibn Ali Al-Baqir (as) was asked about a man who entered the prayer with the Imam and was already behind by one rak'ah. When the Imam completed the prayer and left with the congregation, the man remembered that he had missed one rak'ah. Imam (as) said: "He should repeat one rak'ah."

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

Hadith.1021 - Abdurrahman ibn Al-Hajjaj narrated from Abu Ibrahim who said: "I asked Imam Jafar ibn Muhammad Al-Sadiq (as) about a man who does not know whether he prayed two, three, or four rak'ahs. Imam (as) said: 'He should pray two rak'ahs standing, then make salaam (salutation), then pray two more rak'ahs while sitting.'

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

Hadith.1022 - It was narrated from Ali ibn Abi Hamza that he asked the Righteous Servant (Imam Musa al-Kadhim (as)) about a man who doubts and does not know whether he has prayed one, two, three, or four units (rak'ahs) of prayer. His prayer becomes confusing for him. Imam said: "All of that [doubt] applies." I said: "Yes." Imam (as) replied: "Then let him continue with his prayer and seek refuge in Allah (swt) from the accursed Satan, for it is likely that this [state of doubt] will go away."

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