حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ حَدَّثَنِي مُحَمَّدُ بْنُ إِسْحَاقَ عَنْ مَكْحُولٍ عَنْ كُرَيْبٍ عَنِ ابْنِ عَبَّاسٍ أَنَّهُ قَالَ لَهُ عُمَرُ يَا غُلَامُ هَلْ سَمِعْتَ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَوْ مِنْ أَحَدٍ مِنْ أَصْحَابِهِ إِذَا شَكَّ الرَّجُلُ فِي صَلَاتِهِ مَاذَا يَصْنَعُ قَالَ فَبَيْنَا هُوَ كَذَلِكَ إِذْ أَقْبَلَ عَبْدُ الرَّحْمَنِ بْنُ عَوْفٍ فَقَالَ فِيمَ أَنْتُمَا فَقَالَ عُمَرُ سَأَلْتُ هَذَا الْغُلَامَ هَلْ سَمِعْتَ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَوْ أَحَدٍ مِنْ أَصْحَابِهِ إِذَا شَكَّ الرَّجُلُ فِي صَلَاتِهِ مَاذَا يَصْنَعُ فَقَالَ عَبْدُ الرَّحْمَنِ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ أَوَاحِدَةً صَلَّى أَمْ ثِنْتَيْنِ فَلْيَجْعَلْهَا وَاحِدَةً وَإِذَا لَمْ يَدْرِ ثِنْتَيْنِ صَلَّى أَمْ ثَلَاثًا فَلْيَجْعَلْهَا ثِنْتَيْنِ وَإِذَا لَمْ يَدْرِ أَثَلَاثًا صَلَّى أَمْ أَرْبَعًا فَلْيَجْعَلْهَا ثَلَاثًا ثُمَّ يَسْجُدْ إِذَا فَرَغَ مِنْ صَلَاتِهِ وَهُوَ جَالِسٌ قَبْلَ أَنْ يُسَلِّمَ سَجْدَتَيْنِ
مسند امام احمد بن حنبل · Hadith 1656
It was narrated from Ibn 'Abbas (RA) that 'Umar (RA) said to him: O boy, did you hear from the Messenger of Allah (PBUH) or from one of his Companions anything about what a man should do if he is not sure about his prayer? Whilst he was saying that, 'Abdul Rahman bin 'Awf came and said: What are you talking about? 'Umar said: I asked this boy whether he heard anything from the Messenger of Allah (PBUH) or from one of his Companions about what a man should do if he is not sure about his prayer. 'Abdul Rahman said: I heard the Messenger of Allah (PBUH)say: "If one of you is not sure about his prayer and does not know whether he prayed one or two [rak'ahs], let him assume it is one; and if he does not know whether he prayed two or three, let him assume that it is two; and if he does not know whether he prayed three or four, let him assume that it is three; then let him prostrate twice when he finishes his prayer, whilst he is still sitting, before he says the salam."
Sanad
حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ حَدَّثَنِي مُحَمَّدُ بْنُ إِسْحَاقَ عَنْ مَكْحُولٍ عَنْ كُرَيْبٍ عَنِ ابْنِ عَبَّاسٍ