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
bayhaqi:3964 – Abū ʿAbdullāh al-Ḥāfiẓ > Abū Bakr b. Isḥāq al-Faqīh > ʿAbdullāh b. Muḥammad > Muḥammad b. Yaḥyá > Abū al-Walīd > Abū Wakīʿ > Abū Isḥāq > Abū al-Aḥwaṣ > ʿAbdullāh > al-Tashahhud Tamām al-Ṣalāh Waruwwīnā > ʿUmar b. al-Khaṭṭāb

[Machine] "He said that prayer is not valid except with the tashahhud."  

البيهقي:٣٩٦٤ – وَأَخْبَرَنَا أَبُو عَبْدِ اللهِ الْحَافِظُ أنبأ أَبُو بَكْرِ بْنُ إِسْحَاقَ الْفَقِيهُ أنبأ عَبْدُ اللهِ بْنُ مُحَمَّدٍ ثنا مُحَمَّدُ بْنُ يَحْيَى ثنا أَبُو الْوَلِيدِ ثنا أَبُو وَكِيعٍ عَنْ أَبِي إِسْحَاقَ عَنْ أَبِي الْأَحْوَصِ عَنْ عَبْدِ اللهِ قَالَ التَّشَهُّدُ تَمَامُ الصَّلَاةِ وَرُوِّينَا عَنْ عُمَرَ بْنِ الْخَطَّابِ ؓ

أَنَّهُ قَالَ لَا تَجُوزُ صَلَاةٌ إِلَّا بِتَشَهُّدٍ  

See similar narrations below:

Collected by Suyūṭī