Sunan an-Nasa'i 1163

أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا مُحَمَّدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ أَبَا إِسْحَاقَ، يُحَدِّثُ عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا لاَ نَدْرِي مَا نَقُولُ فِي كُلِّ رَكْعَتَيْنِ غَيْرَ أَنْ نُسَبِّحَ وَنُكَبِّرَ وَنَحْمَدَ رَبَّنَا وَأَنَّ مُحَمَّدًا صلى الله عليه وسلم عُلِّمَ فَوَاتِحَ الْخَيْرِ وَخَوَاتِمَهُ فَقَالَ ‏ "‏ إِذَا قَعَدْتُمْ فِي كُلِّ رَكْعَتَيْنِ فَقُولُوا 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 وَلْيَتَخَيَّرْ أَحَدُكُمْ مِنَ الدُّعَاءِ أَعْجَبَهُ إِلَيْهِ فَلْيَدْعُ اللَّهَ عَزَّ وَجَلَّ ‏"‏ ‏.‏

It was narrated that 'Abdullah said:"We used not to know what we should say in each rak'ah apart from glorifying, magnifying and praising our Lord. But Muhammad (ﷺ) taught us everything about what is good. He said: "When you sit following every two rak'ahs, then 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), then choose any supplication that you like and call upon Allah the Mighty and Sublime with it

Reference

Sunan an-Nasa'i 1163

In-book

Book 12, Hadith 135