Doubt in prayer and two prostrations before salam

[Machine] I heard from the Messenger of Allah ﷺ or from one of his companions what he mentioned that the Messenger of Allah ﷺ commanded when a person makes a mistake in their prayer. I asked: "Have you heard anything about this, O Amir al-Mumineen?" He replied: "No." Then Abdullah ibn Awf entered upon us and asked: "What are you discussing?" Umar said: "I asked him if he heard the Messenger of Allah ﷺ or any of his companions mention what the Messenger of Allah ﷺ commanded when a person makes a mistake in their prayer." Abdullah ibn Awf said: "I have knowledge of that." Umar said: "Come forth and narrate, for you are the just and trustworthy." Abdullah ibn Awf said: "I heard the Messenger of Allah ﷺ say, 'If one of you is uncertain between two scenarios [in prayer], then they should choose the one that is less in number. And if one is uncertain between two scenarios and a third, they should choose the two. And if one is uncertain between three scenarios and four, they should choose the three. Then they should complete what remains of their prayer until the doubt only pertains to the extra [acts of worship]. Then they should prostrate twice while sitting before completing their prayer.'"

سَمِعْتَ مِنْ رَسُولِ اللَّهِ ﷺ أَوْ مِنْ أَحَدٍ مِنْ أَصْحَابِهِ مَا يَذْكُرُ مَا أَمَرَ بِهِ رَسُولُ اللَّهِ ﷺ إِذَا سَهَا الْمَرْءُ فِي صَلَاتِهِ؟ قُلْتُ لَا أَوَمَا سَمِعْتَ يَا أَمِيرَ الْمُؤْمِنِينَ؟ قَالَ لَا فَدَخَلَ عَلَيْنَا عَبْدُ الرَّحْمَنِ بْنُ عَوْفٍ فَقَالَ فِيمَا أَنْتُمَا؟ فَقَالَ عُمَرُ سَأَلْتُهُ هَلْ سَمِعَ رَسُولَ اللَّهِ ﷺ أَوْ مِنْ أَحَدٍ مِنْ أَصْحَابِهِ يَذْكُرُ مَا أَمَرَ بِهِ رَسُولُ اللَّهِ ﷺ إِذَا سَهَا الْمَرْءُ فِي صَلَاتِهِ فَقَالَ عَبْدُ الرَّحْمَنِ عِنْدِي عِلْمٌ مِنْ ذَلِكَ فَقَالَ عُمَرُ هَلُمَّ فَأَنْتَ الْعَدْلُ الرِّضَا فَقَالَ عَبْدُ الرَّحْمَنِ سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ «إِذَا شَكَّ أَحَدُكُمْ فِي الِاثْنَتَيْنِ فَلْيَجْعَلْهُمَا وَاحِدَةً وَإِذَا شَكَّ فِي الِاثْنَتَيْنِ وَالثَّلَاثِ فَلْيَجْعَلْهُمَا اثْنَتَيْنِ وَإِذَا شَكَّ فِي الثَّلَاثِ وَالْأَرْبَعِ فَلْيَجْعَلْهُمَا ثَلَاثًا ثُمَّ يُتِمُّ مَا بَقِيَ مِنْ صَلَاتِهِ حَتَّى يَكُونَ الْوَهْمُ فِي الزِّيَادَةِ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ أَنْ يُسَلِّمَ»

[Machine] I sat with Umar bin Al-Khattab, and he said: "O Ibn Abbas, have you heard from the Prophet ﷺ about a person who forgets their prayer and does not know whether they increased or decreased in it?" I said: "And have you heard anything about it, O Amir al-Mu'minin, from the Messenger of Allah ﷺ ?" He said: "No, by Allah, I have not heard anything from him regarding it, nor have I asked him about it. But when Abdur-Rahman bin Awf came, Umar informed him about it. And Abdur-Rahman said: 'But I have heard this from the Prophet ﷺ .' Umar said: 'Then you have the knowledge that we were seeking, so what did you hear?' Abdur-Rahman said: 'I heard the Prophet ﷺ say: If any one of you is in doubt about his prayer, then he should consider it as one unit (rak'ah) and two units (rak'ah), and if he is in doubt about two units (rak'ah) or three, then he should consider it as two units (rak'ah), and if he is in doubt about three or four, then he should consider it as three units (rak'ah), until the doubt concerning the addition is removed, and he should prostrate twice before he gives the Salam, then he should give the Salam, and this is its meaning.'"

جَلَسْتُ إِلَى عُمَرَ بْنِ الْخَطَّابِ ؓ فَقَالَ يَا ابْنَ عَبَّاسٍ هَلْ سَمِعْتَ مِنَ النَّبِيِّ ﷺ فِي الرَّجُلِ إِذَا نَسِيَ صَلَاتَهَ فَلَمْ يَدْرِ أَزَادَ أَمْ نَقَصَ مَا أَمَرَ بِهِ فِيهِ؟ قُلْتُ وَمَا سَمِعْتَ أَنْتَ يَا أَمِيرَ الْمُؤْمِنِينِ مِنْ رَسُولِ اللهِ ﷺ شَيْئًا فِي ذَلِكَ؟ قَالَ لَا وَاللهِ مَا سَمِعْتُ مِنْهُ فِيهِ شَيْئًا وَلَا سَأَلْتُ عَنْهُ إِذْ جَاءَ عَبْدُ الرَّحْمَنِ بْنُ عَوْفٍ فَقَالَ فِيمَا أَنْتُمَا؟ فَأَخْبَرَهُ عُمَرُ فَقَالَ سَأَلْتُ هَذَا الْفَتَى عَنْ كَذَا وَكَذَا فَلَمْ أَجِدْ عِنْدَهُ عِلْمًا قَالَ عَبْدُ الرَّحْمَنِ لَكِنْ عِنْدِي لَقَدْ سَمِعْتُ ذَلِكَ مِنَ النَّبِيِّ ﷺ فَقَالَ عُمَرُ فَأَنْتَ عِنْدَنَا الْعَدْلُ الرِّضَا فَمَاذَا سَمِعْتَ؟ قَالَ سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَشَكَّ فِي الْوَاحِدَةِ وَالِثْنَتَيْنِ فَلْيَجْعَلْهُمَا وَاحِدَةً وَإِذَا شَكَّ فِي الِاثْنَتَيْنِ وَالثَّلَاثِ فَلْيَجْعَلْهَا اثْنَتَيْنِ وَإِذَا شَكَّ فِي الثَّلَاثِ وَالْأَرْبَعِ فَلْيَجْعَلْهَا ثَلَاثًا حَتَّى يَكُونَ الْوَهْمُ فِي الزِّيَادَةِ يَسْجُدُ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ ثُمَّ يُسَلِّمُ وَبِمَعْنَاهُ