"I did not see Messenger of Allah (S) praying voluntary prayers sitting until the year before he died. Then he would perform Salat for the voluntary prayers sitting, and he would recite a Surah and prolong it such that it would be longer than the longest of them."  

عَنْ حَفْصَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ صَلَّى فِي سُبْحَتِهِ قَاعِدًا حَتَّى كَانَ قَبْلَ وَفَاتِهِ ﷺ بِعَامٍ فَإِنَّهُ كَانَ يُصَلِّي فِي سُبْحَتِهِ قَاعِدًا وَيَقْرَأُ بِالسُّورَةِ وَيُرَتِّلُهَا حَتَّى تَكُونَ أَطْوَلَ مِنْ أَطْوَلَ مِنْهَا وَفِي الْبَابِ عَنْ أُمِّ سَلَمَةَ وَأَنَسِ بْنِ مَالِكٍ  

"The Prophet (S) would perform Salat while sitting. And he would recite while sitting. When about thirty or forty Ayah of his recitation remained he would standup and recite while he was standing, then he would bow and prostrate. Then he would do the same in the second Rak'ah."  

أَنَّ النَّبِيَّ ﷺ كَانَ يُصَلِّي جَالِسًا فَيَقْرَأُ وَهُوَ جَالِسٌ فَإِذَا بَقِيَ مِنْ قِرَاءَتِهِ قَدْرُ مَا يَكُونُ ثَلاَثِينَ أَوْ أَرْبَعِينَ آيَةً قَامَ فَقَرَأَ وَهُوَ قَائِمٌ ثُمَّ رَكَعَ وَسَجَدَ ثُمَّ صَنَعَ فِي الرَّكْعَةِ الثَّانِيَةِ مِثْلَ ذَلِكَ  

"I asked her about the voluntary Salat of Messenger of Allah (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.'"  

عَنْ صَلاَةِ رَسُولِ اللَّهِ ﷺ 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