Sahih Muslim 731
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah
Alqama b. Waqqas reported:
وَحَدَّثَنَا ابْنُ نُمَيْرٍ ، حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ ، حَدَّثَنَا مُحَمَّدُ بْنُ عَمْرٍو ، حَدَّثَنِي مُحَمَّدُ بْنُ إِبْرَاهِيمَ ، عَنْ عَلْقَمَةَ بْنِ وَقَّاصٍ ، قَالَ قُلْتُ لِعَائِشَةَ كَيْفَ كَانَ يَصْنَعُ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الرَّكْعَتَيْنِ وَهُوَ جَالِسٌ قَالَتْ كَانَ يَقْرَأُ فِيهِمَا فَإِذَا أَرَادَ أَنْ يَرْكَعَ قَامَ فَرَكَعَ .
I asked 'A'isha how the Messenger of Allah (ﷺ) did in the two rak'ahs as he (observed them) sitting. She said: He would recite (the Qur'an) in them, and when he intended to bow, he would stand up and then bowed.