Sunan an-Nasai Book 12, Hadith 1166

أَخْبَرَنَا أَحْمَدُ بْنُ عَمْرِو بْنِ السَّرْحِ، قَالَ حَدَّثَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي عَمْرُو بْنُ الْحَارِثِ، أَنَّ زَيْدَ بْنَ أَبِي أُنَيْسَةَ الْجَزَرِيَّ، حَدَّثَهُ أَنَّ أَبَا إِسْحَاقَ حَدَّثَهُ عَنِ الأَسْوَدِ، وَعَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، قَالَ كُنَّا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم لاَ نَعْلَمُ شَيْئًا فَقَالَ لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ قُولُوا فِي كُلِّ جَلْسَةٍ التَّحِيَّاتُ لِلَّهِ وَالصَّلَوَاتُ وَالطَّيِّبَاتُ السَّلاَمُ عَلَيْكَ أَيُّهَا النَّبِيُّ وَرَحْمَةُ اللَّهِ وَبَرَكَاتُهُ السَّلاَمُ عَلَيْنَا وَعَلَى عِبَادِ اللَّهِ الصَّالِحِينَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ ‏"‏ ‏.‏

It was narrated from Al-Aswad and 'Alqamah, that Abdullah bin Mas'ud said:"We were with the Messenger of Allah (ﷺ) and we did not know anything, then the Messenger of Allah (ﷺ) said to us: "Every time you sit (in prayer), say: At-tahiyyatu lillahi was-salawatu wat-tayyibat, as-salamu 'alaika ayyuhan-Nabiyyu wa rahmatAllahi wa baraktuhu. As-salamu 'alaina wa 'ala 'ibad illahis-salihin, ashahdu an la illaha ill-Allah wa ashhadu anna Muhammadan 'abduhu wa rasuluhu (Allah compliments, prayers and pure words are due to Allah. Peace be upon you, O Prophet, and the mercy of Allah (SWT) and his blessings. Peace be upon us and upon the righteous slaves of Allah (SWT). I bear witness that none has the right to be worshipped except Allah and I bear witness that Muhammad is His slave and Messenger