Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that he asked Sayyidah Aishah (RA) about the salah of Allah's Messenger the supererogatory thereof. She said that he used to pray long into the night, standing up. And (he used to pray) long into the night, sitting down. If he recited while he was standing then he went into ruku and sajdah from there. And if he recited while he was seated then he went into ruku and sajdah from there. [Ahmed 24074, Muslim 730, Abu Dawud 1228]