The Book of Prayer - Travellers
6/16. Chapter:
It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah
Sahih Muslim
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا يَزِيدُ بْنُ زُرَيْعٍ، عَنْ سَعِيدٍ الْجُرَيْرِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، قَالَ قُلْتُ لِعَائِشَةَ هَلْ كَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي وَهُوَ قَاعِدٌ قَالَتْ نَعَمْ بَعْدَ مَا حَطَمَهُ النَّاسُ .
'Abdullah b. Shaqiq reported: I asked 'A'isha whether the Messenger of Allah (ﷺ) observed (Nafl) sitting. She said: Yes, when the people had made him old.
Reference : Sahih Muslim 732aIn-book reference : Book 6, Hadith 138USC-MSA web (English) reference : Book 4, Hadith 1593