Sahih Muslim Book - 6, Hadith - 1705

وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ عَبْدِ اللَّهِ بْنِ يَزِيدَ وَأَبِى النَّضْرِ عَنْ أَبِى سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ عَنْ عَائِشَةَ أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- كَانَ يُصَلِّى جَالِسًا فَيَقْرَأُ وَهُوَ جَالِسٌ فَإِذَا بَقِىَ مِنْ قِرَاءَتِهِ قَدْرُ مَا يَكُونُ ثَلاَثِينَ أَوْ أَرْبَعِينَ آيَةً قَامَ فَقَرَأَ وَهُوَ قَائِمٌ ثُمَّ رَكَعَ ثُمَّ سَجَدَ ثُمَّ يَفْعَلُ فِى الرَّكْعَةِ الثَّانِيَةِ مِثْلَ ذَلِكَ.

It was narrated from 'Aishah that the Messenger of Allah (s.a.w) used to pray sitting, and he would recite while sitting, then when there were about thirty or forty verses left of his recitation, he would stand up and recite while standing. Then he would bow, then prostrate, then he would do likewise in the second Rak'ah.