✅Answer: Please pay attention to the following narrations:

عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: إِذَا لَمْ تَدْرِ ثَلَاثاً صَلَّيْتَ أَوْ أَرْبَعاً وَ وَقَعَ رَأْيُكَ عَلَى الثَّلَاثِ فَابْنِ عَلَى الثَّلَاثِ وَ إِنْ وَقَعَ رَأْيُكَ عَلَى الْأَرْبَعِ فَسَلِّمْ وَ انْصَرِفْ وَ إِنِ اعْتَدَلَ وَهْمُكَ فَانْصَرِفْ وَ صَلِّ رَكْعَتَيْنِ وَ أَنْتَ جَالِسٌ

📚 Al-Kafi, Vol. 3, p. 353

Translation:

Imam Sadiq (peace be upon him) said: If you do not know whether you have performed three rak'ahs or four rak'ahs and your inclination and opinion are towards three rak'ahs, then base it on three rak'ahs. And if your inclination and opinion are towards four rak'ahs, then say the salam and finish the prayer. And if your doubt is equal (meaning your opinion on three rak'ahs is the same as your opinion on four rak'ahs), then finish the prayer and perform two rak'ahs of prayer while sitting (after the prayer).

عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: فِيمَنْ لَا يَدْرِي أَ ثَلَاثاً صَلَّى أَمْ أَرْبَعاً وَ وَهْمُهُ فِي ذَلِكَ سَوَاءٌ قَالَ فَقَالَ إِذَا اعْتَدَلَ الْوَهْمُ فِي الثَّلَاثِ وَ الْأَرْبَعِ فَهُوَ بِالْخِيَارِ إِنْ شَاءَ صَلَّى رَكْعَةً وَ هُوَ قَائِمٌ وَ إِنْ شَاءَ صَلَّى رَكْعَتَيْنِ وَ أَرْبَعَ سَجَدَاتٍ وَ هُوَ جَالِسٌ

📚 Al-Kafi, Vol. 3, p. 353

Translation: Imam Sadiq (peace be upon him) said about a person who does not know whether he has performed three rak'ahs or four rak'ahs and his doubt and perception about this are equal:

If his doubt and perception about three rak'ahs and four rak'ahs are equal, he has a choice and is free to, if he wishes, perform one rak'ah of prayer while standing, or perform two rak'ahs of prayer while sitting with four prostrations.

عَنْ أَبِي عَبْدِ اللَّهِ فِي حَدِيثٍ قَالَ: إِذَا كُنْتَ لَا تَدْرِي ثَلَاثاً صَلَّيْتَ أَمْ أَرْبَعاً وَ لَمْ يَذْهَبْ وَهْمُكَ إِلَى شَيْ ءٍ فَسَلِّمْ ثُمَّ صَلِّ رَكْعَتَيْنِ وَ أَنْتَ جَالِسٌ تَقْرَأُ فِيهِمَا بِأُمِّ الْكِتَابِ- وَ إِنْ ذَهَبَ وَهْمُكَ إِلَى الثَّلَاثِ فَقُمْ فَصَلِّ الرَّكْعَةَ الرَّابِعَةَ وَ لَا تَسْجُدْ سَجْدَتَيِ السَّهْوِ فَإِنْ ذَهَبَ وَهْمُكَ إِلَى الْأَرْبَعِ فَتَشَهَّدْ وَ سَلِّمْ ثُمَّ اسْجُدْ سَجْدَتَيِ السَّهْوِ

📚 Al-Kafi, Vol. 3, p. 353

Translation:

Imam Sadiq (peace be upon him) said in a hadith: If you did not know whether you have performed three rak'ahs or four rak'ahs and your doubt and perception did not lean towards anything (and were equal), then say the salam (i.e., base it on four rak'ahs and finish the prayer and say the salam) and perform two rak'ahs of prayer while sitting, in which you recite only Surah Al-Hamd. And if your doubt and perception leaned more towards three rak'ahs, then get up and perform the fourth rak'ah and do not perform two prostrations of forgetfulness. And if your doubt and perception leaned more towards four rak'ahs (base it on four rak'ahs) and finish the prayer and say the salam, and after that, perform two prostrations of forgetfulness.

✅It is completely clear from the above narrations that if we have doubt in a four-rak'ah prayer (Dhuhr, Asr, and Isha) whether we have performed three rak'ahs or four rak'ahs, there are three possible scenarios:

✅1- If our doubt and perception lean more towards having performed three rak'ahs, we base it on three rak'ahs and perform one more rak'ah and say the salam and we have no other duty.

✅2- If our doubt and perception lean more towards four rak'ahs, we base it on four rak'ahs and say the salam and perform two prostrations of forgetfulness after the prayer.

✅3- If our doubt and perception are equal, meaning our doubt and perception towards three rak'ahs are the same as towards four rak'ahs, here we base it on four rak'ahs and finish the prayer, and after the prayer, we either perform two rak'ahs of prayer while sitting or one rak'ah of prayer while standing, in which we recite only Al-Hamd without a Surah.

✅Note 1: The above hadiths are specific to four-rak'ah prayers, namely Dhuhr, Asr, and Isha.

✅Note 2: The rulings for a person with frequent doubt, who has a specific definition in narrations, are different, and these above narrations are for ordinary people.

Doubt in a four-rak'ah prayer

Doubts in prayer