Holy Prophet’s (sa) tahajjud in old age

عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ مَا رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم 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.‏

Hazrat Aishah^ra related: “I never saw the Prophet^sa recite the Quran seated during any part of his night-prayer, until age weighed upon him. Thereafter, he would recite while sitting and when roughly thirty or forty verses of a Surah remained, he would rise to his feet, recite those verses standing and then bow in ruku‘.”

(Sahih al-Bukhari, Kitab at-tahajjud, Bab qiyami n-nabiyyi^sa bi l-layli fi ramadana wa ghayrih, Hadith 1148)