(16) باب جَوَازِ النَّافِلَةِ قَائِمًا وَقَاعِدًا وَفِعْلِ بَعْضِ الرَّكْعَةِ قَائِمًا وَبَعْضِهَا قَاعِدًا
'Abdullah b. Shaqiq reported:
I asked 'A'isha whether the Messenger of Allah (ﷺ) observed (Nafl) sitting. She said: Yes, when the people had made him old.
Reference: Sahih Muslim 732 a
In-book reference: Book 6, Hadith 138
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1593