"I asked her about the voluntary Salat of Allah's Messenger (S), she said: 'He would pray long into the night standing, and long into the night sitting. So when he recited and he was standing, he would bow and prostrate whole he was standing, and when he recited while he was sitting, he bowed and prostrated while he was sitting.'"
حَدَّثَنَا أَحْمَدُ بْنُ مَنِيعٍ، حَدَّثَنَا هُشَيْمٌ، أَخْبَرَنَا خَالِدٌ، وَهُوَ الْحَذَّاءُ عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، عَنْ عَائِشَةَ، قَالَ سَأَلْتُهَا عَنْ صَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم عَنْ تَطَوُّعِهِ قَالَتْ كَانَ يُصَلِّي لَيْلاً طَوِيلاً قَائِمًا وَلَيْلاً طَوِيلاً قَاعِدًا فَإِذَا قَرَأَ وَهُوَ قَائِمٌ رَكَعَ وَسَجَدَ وَهُوَ قَائِمٌ وَإِذَا قَرَأَ وَهُوَ جَالِسٌ رَكَعَ وَسَجَدَ وَهُوَ جَالِسٌ . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ .
Tirmizi
The Book on Salat (Prayer) • Hadith 375