✅Answer: Please pay attention to the following narrations:

1 عن زرارة قال: قُلْتُ لِأَبِی عَبْداللَّهِ رَجُلٌ شَکَّ فی الْأَذَانِ و قَدْ دَخَلَ فی الْإِقَامَةِ قال یَمْضِی قُلْتُ رَجُلٌ شَکَّ فِی الْأَذَانِ و الْإِقَامَةِ و قَدْ کَبَّرَ قَالَ یَمْضِی قُلْتُ رَجُلٌ شَکَّ فی التَّکْبِیرِ وَ قَدْ قَرَأَ قَالَ یَمْضِی قُلْتُ شَکَّ فِی الْقِرَاءَةِ و قَدْ رَکَعَ قَالَ یَمْضِی قُلْتُ شَکَّ فی الرُّکُوعِ وَ قَدْ سَجَدَ قَالَ یَمْضِی عَلَی صَلَاتِهِ ثُمَّ قال یا زُرَارَةُ إِذا خَرَجْتَ مِنْ شَیْءٍ ثُمَّ دَخَلْتَ فی غَیْرِهِ فَشَکُّکَ لَیْسَ بِشَیْءٍ

📚Tahdhib, Vol. 2, p. 352

Zurara said: 'I said to Imam Sadiq (peace be upon him): 'A person doubts about the adhan while he has entered the iqamah?'' He said: 'He should proceed!'

I said: 'A person doubts about the adhan and iqamah while he has said the takbirat al-ihram (the first takbir to start the prayer)?' He said: 'He should proceed!'

I said: 'A person doubts about the takbirat al-ihram while he has entered the recitation?' He said: 'He should proceed!'

I said: 'He doubts about the recitation while he has gone into bowing?' He said: 'He should proceed!'

I said: 'He doubts about the bowing while he has gone into prostration?' He said: 'He should continue his prayer!'

Then he said: 'O Zurara! When you have left something and entered into something else, your doubt (about the previous one) is nothing!'

2 عن محمد بن مسلم عن أَبی جعفر قال: کُلُّ مَا شَکَکْتَ فِیهِ مِمَّا قَدْ مَضَی فَامْضِهِ کَمَا هُوَ

📚Tahdhib, Vol. 2, p. 344

Imam Baqir (peace be upon him) said: 'Whatever you doubt about that has passed (its place), pass by it and pay no attention to it!'

3 عن إِسْمَاعِيلَ بْنِ جَابِرٍ عن أَبِي عَبْداللَّه في رَجُلٍ نَسِيَ أَنْ يَسْجُدَ سَجْدَةَ الثَّانِيَةِ حَتَّى قَامَ فَذَكَرَ وَ هُوَ قَائِمٌ أَنَّهُ لَمْ يَسْجُدْ قَالَ فَلْيَسْجُدْ مَا لَمْ يَرْكَعْ فَإِذَا رَفَعَ [رَکَعَ] فَذَكَرَ بَعْدَ رُكُوعِهِ أَنَّهُ لَمْ يَسْجُدْ فَلْيَمْضِ عَلَى صَلَاتِهِ حَتَّى يُسَلِّمَ ثُمَّ يَسْجُدُهَا فَإِنَّهَا قَضَاءٌ وَ قَالَ قَالَ أَبُو عَبْدِاللَّهِ إِنْ شَكَّ فِي الرُّكُوعِ بَعْدَ مَا سَجَدَ فَلْيَمْضِ و إِنْ شَكَّ فِي السُّجُودِ بَعْدَ مَا قَامَ فَلْيَمْضِ كُلُّ شَيْءٍ شَكَّ فِيهِ مِمَّا قَدْ جَاوَزَهُ وَ دَخَلَ فِي غَيْرِهِ فَلْيَمْضِ عَلَيْه

📚Tahdhib, Vol. 2, p. 153

Imam Sadiq (peace be upon him) said about a person who forgets to perform the second prostration until he stands up and while standing, remembers that he has not performed the prostration: 'As long as he has not gone into bowing, he must perform that prostration, but if he has gone into bowing and after going into bowing he remembers that he has not performed (one) prostration, he should continue his prayer until he gives the salam and after the salam of the prayer, he should perform that prostration, for indeed it is considered a make-up.'

And Imam Sadiq (peace be upon him) said: 'If he doubts about bowing after he has gone into prostration, he should pass by it and pay no attention to it, and if he doubts about prostration after he has stood up, he should pass by it and pay no attention to it. Whatever you doubt about that has passed (its place) and you have entered into something else, pass by it and pay no attention to it!'

4️⃣Narrated by Abdulrahman ibn Abi Abdullah: I said to Imam Sadiq (peace be upon him): 'A man raises his head from prostration and doubts before he is fully seated, and he does not know if he has prostrated or not?' He said: 'He should prostrate.' I said: 'A man rises from his prostration and doubts before he is fully standing, and he does not know if he has prostrated or not?' He said: 'He should prostrate!'

📚Tahdhib, Vol. 2, p. 153

Abdulrahman said: 'I said to Imam Sadiq (peace be upon him): 'A man raises his head from prostration and before he is fully seated, he doubts and does not know whether he has prostrated or not?' The Imam said: 'He should prostrate!' I said: 'A person rises from prostration and before he is completely standing, he doubts whether he has prostrated or not?' He said: 'He should prostrate!'

✅Based on the above and other narrations, if a person doubts about one of the actions of the prayer (not the rak'ahs) after its time has passed and he has entered into another action, he should not pay attention to his doubt and should continue his prayer, and there is no duty upon him.

Therefore, if a person is in the standing position and doubts about the prostration of the previous rak'ah, he should not pay attention to his doubt and should continue his prayer, and there is no duty upon him.

However, if his doubt about the prostration is before he gets up and stands completely straight and reaches the standing position, he should perform the prostration to be certain.

✅According to the above and other narrations, if he becomes certain that he has omitted a prostration before going into the bowing of the next rak'ah, he should go back and perform the prostration and continue the rest of the prayer. But if he has passed the bowing and becomes certain that he has omitted one prostration from the previous rak'ah, he should continue the prayer and perform that prostration as a make-up after the prayer.

https://feghh14.com/en