صَحِيحُ مُسْلِمٍ
Sahih Muslim 732 a
The Book of Prayer - Travellers
It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah
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.
Sahih Muslim, 732 aSahihصحيح