(16) باب جَوَازِ النَّافِلَةِ قَائِمًا وَقَاعِدًا وَفِعْلِ بَعْضِ الرَّكْعَةِ قَائِمًا وَبَعْضِهَا قَاعِدًا
Alqama b. Waqqas reported:
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.
Reference: Sahih Muslim 731 d
In-book reference: Book 6, Hadith 137
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1592