Sunan an-Nasa'i, Hadith 1646 ⓘ
It was narrated that 'Aishah said: "The Messenger of Allah used to pray for a long time at night. If he started to pray standing, he would bow standing and if he started to pray sitting, he would bow sitting."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ بُدَيْلٍ، وَأَيُّوبَ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي لَيْلاً طَوِيلاً فَإِذَا صَلَّى قَائِمًا رَكَعَ قَائِمًا وَإِذَا صَلَّى قَاعِدًا رَكَعَ قَاعِدًا .
also referenced under: Book 20, Hadith 49 (In-Book) & Volume 2, Book 20, Hadith 1647 (DarusSalam)