Sahih Muslim 730
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
'Abdullah b. Shaqiq al-'Uqaili reported:
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، أَخْبَرَنَا أَبُو مُعَاوِيَةَ ، عَنْ هِشَامِ بْنِ حَسَّانَ ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ الْعُقَيْلِيِّ ، قَالَ سَأَلْنَا عَائِشَةَ عَنْ صَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُكْثِرُ الصَّلاَةَ قَائِمًا وَقَاعِدًا فَإِذَا افْتَتَحَ الصَّلاَةَ قَائِمًا رَكَعَ قَائِمًا وَإِذَا افْتَتَحَ الصَّلاَةَ قَاعِدًا رَكَعَ قَاعِدًا .
I asked 'A'isha about the prayer of the Messenger of Allah (ﷺ). She said: The Messenger of Allah (ﷺ) would observe prayer (Nafl) in a standing position as well as in a sitting position, and when he commenced the prayer in a standing position, he bowed in this very position, and when he commenced the prayer in a sitting position, he bowed in this very position.