(16) باب جَوَازِ النَّافِلَةِ قَائِمًا وَقَاعِدًا وَفِعْلِ بَعْضِ الرَّكْعَةِ قَائِمًا وَبَعْضِهَا قَاعِدًا
'A'isha reported:
The Messenger of Allah (ﷺ) used to pray while sitting (when he grew old) and he recited in this position and when the recitation equal to thirty or forty verses was left, he would then stand up and recite (for this duration) in a standing position and then bowed himself and then prostrated himself and did the same in the second rak'ah.
Reference: Sahih Muslim 731 b
In-book reference: Book 6, Hadith 135
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1590