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
suyuti:2-1398b – ʿMr > Dhā Rfʿ Ḥdukm Rʾsh Wẓn
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السيوطي:٢-١٣٩٨b –

"عن عمرَ قال: إذا رفعَ أحدُكم رأسهُ وظنَّ أن الإمامَ قد رفَع فَلْيُعِد رأسهُ فإذا رفع الإمامُ رأسهُ فليمكث قدرَ ما تركَ".  

[ق] البيهقى في السنن

See similar narrations below:

Collected by Bayhaqī
bayhaqi:2602 – Abū Bakr b. al-Ḥārith > Abū Muḥammad b. Ḥayyān > Ibrāhīm b. Muḥammad b. al-Ḥasan > Abū ʿĀmir > al-Walīd > Ibn Lahīʿah > Bukayr b. ʿAbdullāh b. al-Ashaj > al-Ḥārith b. Makhlad from his father > ʿUmar b. al-Khaṭṭāb > Idhā Rafaʿ Aḥadukum Raʾsah Faẓan

[Machine] "That the Imam has lifted up, so let him put his head down. When the Imam lifts his head up, let him stay for as long as he left. It has been narrated to us from Sha'bi and Ibrahim al-Nakha'i that he returns and prostrates."  

البيهقي:٢٦٠٢ – أَخْبَرَنَا أَبُو بَكْرِ بْنُ الْحَارِثِ أنبأ أَبُو مُحَمَّدِ بْنُ حَيَّانَ ثنا إِبْرَاهِيمُ بْنُ مُحَمَّدِ بْنِ الْحَسَنِ ثنا أَبُو عَامِرٍ ثنا الْوَلِيدُ أَخْبَرَنِي ابْنُ لَهِيعَةَ عَنْ بُكَيْرِ بْنِ عَبْدِ اللهِ بْنِ الْأَشَجِّ عَنِ الْحَارِثِ بْنِ مَخْلَدٍ عَنْ أَبِيهِ أَنَّهُ سَمِعَ عُمَرَ بْنَ الْخَطَّابِ يَقُولُ إِذَا رَفَعَ أَحَدُكُمْ رَأْسَهُ فَظَنَّ

أَنَّ الْإِمَامَ قَدْ رَفَعَ فَلْيُعِدْ رَأْسَهُ فَإِذَا رَفَعَ الْإِمَامُ رَأْسَهُ فَلْيَمْكُثْ قَدْرَ مَا تَرَكَ وَرُوِّينَا عَنِ الشَّعْبِيِّ وَإِبْرَاهِيمَ النَّخَعِيِّ أَنَّهُ يَعُودُ فَيَسْجُدُ