(16) باب جَوَازِ النَّافِلَةِ قَائِمًا وَقَاعِدًا وَفِعْلِ بَعْضِ الرَّكْعَةِ قَائِمًا وَبَعْضِهَا قَاعِدًا
'Abdullah b. Shaqiq al-'Uqaili reported:
I asked 'A'isha about the prayer of the Messenger of Allah (ﷺ) during the night (i. e. Tahajjud prayer) She replied: He used to pray for a long time standing and for a long time sitting in the night, and when he recited the Qur'an while standing, he would bow himself from the standing position, and when he recited while sitting, he would bow from the sitting position.
Reference: Sahih Muslim 730 d
In-book reference: Book 6, Hadith 132
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1587