Malik b. al-Huwairith saw that the prophet (may peace by upon him) would not stand at the end of the first or the third rak’ah until he sat down straight.
أَنَّهُ رَأَى النَّبِيَّ ﷺ إِذَا كَانَ فِي وِتْرٍ مِنْ صَلاَتِهِ لَمْ يَنْهَضْ حَتَّى يَسْتَوِيَ قَاعِدًا