Shama'il at-Tirmidhi 18

“I asked 'A’isha (may Allah be well pleased with her) about the voluntary ritual prayers of Allah’s Messenger (Allah bless him and give him peace), and she said: ‘He used to spend a long night performing the ritual prayer while standing, and a long night while sitting, so if he recited the Qur’an while standing, he would bow and prostrate from a standing position, and if he recited while sitting, he would bow and prostrate from a sitting position.”