مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 796
Prayer
The Nature of Prayer - Section 1
وَعَنْهُ أَنَّهُ رَأَى النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي فَإِذَا كَانَ فِي وِتْرٍ مِنْ صَلَاتِهِ لَمْ يَنْهَضْ حَتَّى يَسْتَوِيَ قَاعِدًا. رَوَاهُ البُخَارِيّ
He said that he saw the Prophet (ﷺ) praying, and when he had prayed an odd number of rak'as he did not stand up till after he had adopted a sitting position. Bukhari transmitted it.
Mishkat al-Masabih, 796صَحِيح (الألباني)