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:1247 – Sūwayd b. Naṣr > ʿAbdullāh > Ibn ʿAwn > Ibrāhīm

"They used to say: 'If one is not sure of what he estimates is correct, then prostrate twice.'"  

النسائي:١٢٤٧ – أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ عَنِ ابْنِ عَوْنٍ عَنْ إِبْرَاهِيمَ قَالَ

كَانُوا يَقُولُونَ إِذَا أَوْهَمَ يَتَحَرَّى الصَّوَابَ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ  

See similar narrations below:

Collected by Nasāʾī, Nasāʾī's Kubrá
nasai:1246 – Sūwayd b. Naṣr > ʿAbdullāh > Misʿar > al-Ḥakam > Abū Wāʾil > ʿAbdullāh

"Whoever has doubt, or is not sure, let him estimate what is correct, then let him prostrate twice."  

النسائي:١٢٤٦ – أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ عَنْ مِسْعَرٍ عَنِ الْحَكَمِ عَنْ أَبِي وَائِلٍ عَنْ عَبْدِ اللَّهِ قَالَ

مَنْ شَكَّ أَوْ أَوْهَمَ فَلْيَتَحَرَّ الصَّوَابَ ثُمَّ لْيَسْجُدْ سَجْدَتَيْنِ  

nasai-kubra:1171 – Sūwayd > ʿAbdullāh > Ibn ʿAwn > Ibrāhīm

"They used to say: 'If one is not sure of what he estimates is correct, then prostrate twice.'" (Using translation from Nasāʾī 1247)  

الكبرى للنسائي:١١٧١ – أَخْبَرَنَا سُوَيْدُ قَالَ أَخْبَرَنَا عَبْدُ اللهِ عَنِ ابْنِ عَوْنٍ عَنْ إِبْرَاهِيمَ قَالَ

كَانُوا يَقُولُونَ «إِذَا أَوْهَمَ يَتَحَرَّى الصَّوَابَ ثُمَّ يَسْجُدَ سَجْدَتَيْنِ»