I did not see the Prophet ﷺ reciting (the Qur'an) in the night prayer while sitting except when he became old; when he used to recite while sitting, and when thirty or forty verses remained from the Sura, he would get up and recite them and then bow.  

مَا رَأَيْتُ النَّبِيَّ ﷺ يَقْرَأُ فِي شَىْءٍ مِنْ صَلاَةِ اللَّيْلِ جَالِسًا حَتَّى إِذَا كَبِرَ قَرَأَ جَالِسًا فَإِذَا بَقِيَ عَلَيْهِ مِنَ السُّورَةِ ثَلاَثُونَ أَوْ أَرْبَعُونَ آيَةً قَامَ فَقَرَأَهُنَّ ثُمَّ رَكَعَ  

(the mother of the faithful believers) Messenger of Allah ﷺ (in his last days) used to pray sitting. He would recite while sitting, and when thirty or forty verses remained from the recitation he would get up and recite them while standing and then he would bow and prostrate. He used to do the same in the second rakʿa. After finishing the Prayer he used to look at me and if I was awake he would talk to me and if I was asleep, he would lie down.  

عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ ؓ أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يُصَلِّي جَالِسًا فَيَقْرَأُ وَهْوَ جَالِسٌ فَإِذَا بَقِيَ مِنْ قِرَاءَتِهِ نَحْوٌ مِنْ ثَلاَثِينَ أَوْ أَرْبَعِينَ آيَةً قَامَ فَقَرَأَهَا وَهْوَ قَائِمٌ ثُمَّ يَرْكَعُ ثُمَّ يَسْجُدُ يَفْعَلُ فِي الرَّكْعَةِ الثَّانِيَةِ مِثْلَ ذَلِكَ فَإِذَا قَضَى صَلاَتَهُ نَظَرَ فَإِنْ كُنْتُ يَقْظَى تَحَدَّثَ مَعِي وَإِنْ كُنْتُ نَائِمَةً اضْطَجَعَ