Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
nasai:1240 – Muḥammad b. Rāfiʿ > Yaḥyá b. Ādam > Mufaḍḍal / Ibn Muhalhal > Manṣūr > Ibrāhīm > ʿAlqamah > ʿAbdullāh

"If one of you is not sure about his prayer, let him estimate what he thinks is most likely to be correct and complete the prayer on that basis, then let him prostrate twice."  

النسائي:١٢٤٠ – أَخْبَرَنَا مُحَمَّدُ بْنُ رَافِعٍ قَالَ حَدَّثَنَا يَحْيَى بْنُ آدَمَ قَالَ حَدَّثَنَا مُفَضَّلٌ وَهُوَ ابْنُ مُهَلْهَلٍ عَنْ مَنْصُورٍ عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللَّهِ

يَرْفَعُهُ إِلَى النَّبِيِّ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّ الَّذِي يَرَى أَنَّهُ الصَّوَابُ فَيُتِمَّهُ ثُمَّ يَعْنِي يَسْجُدُ سَجْدَتَيْنِ وَلَمْ أَفْهَمْ بَعْضَ حُرُوفِهِ كَمَا أَرَدْتُ  

See similar narrations below:

Collected by Ṭabarānī, Nasāʾī's Kubrá
tabarani:9827 – Aḥmad b. Shuʿayb Abū ʿAbd al-Raḥman al-Nasāʾī > Muḥammad b. Rāfiʿ al-Naysābūrī > Yaḥyá b. Ādam > Mufaḍḍal b. Muhalhal > Manṣūr > Ibrāhīm > ʿAlqamah > ʿAbdullāh

[Machine] The Prophet ﷺ said, "If any one of you has doubts about his prayer, he should follow what he thinks is correct and complete it, then make two prostrations."  

الطبراني:٩٨٢٧ – حَدَّثَنَا أَحْمَدُ بْنُ شُعَيْبٍ أَبُو عَبْدِ الرَّحْمَنِ النَّسَائِيُّ أنا مُحَمَّدُ بْنُ رَافِعٍ النَّيْسَابُورِيُّ ثنا يَحْيَى بْنُ آدَمَ عَنْ مُفَضَّلِ بْنِ مُهَلْهَلٍ عَنْ مَنْصُورٍ عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللهِ

أَنَّ النَّبِيَّ ﷺ قَالَ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيَتَحَرَّ الَّذِي يَرَى أَنَّهُ الصَّوَابُ فَلْيُتَمِّمْهُ ثُمَّ يَسْجُدْ سَجْدَتَيْنِ»  

nasai-kubra:1164 – Muḥammad b. Rāfiʿ > Yaḥyá b. Ādam > Mufaḍḍal / Ibn Muhalhil > Manṣūr > Ibrāhīm > ʿAlqamah

"If one of you is not sure about his prayer, let him estimate what he thinks is most likely to be correct and complete the prayer on that basis, then let him prostrate twice." (Using translation from Nasāʾī 1240)   

الكبرى للنسائي:١١٦٤ – أَخْبَرَنِي مُحَمَّدُ بْنُ رَافِعٍ قَالَ حَدَّثَنَا يَحْيَى بْنُ آدَمَ قَالَ حَدَّثَنَا مُفَضَّلٌ وَهُوَ ابْنُ مُهَلْهِلٍ عَنْ مَنْصُورٍ عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ

عَنْ عَبْدِ اللهِ يَرْفَعُهُ إِلَى النَّبِيِّ ﷺ قَالَ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيَتَحَرَّ الَّذِي يَرَى أَنَّهُ الصَّوَابُ فَيُتِمَّهُ ثُمَّ يَعْنِي يَسْجُدُ سَجْدَتَيْنِ»