He saw that the Prophet ﷺ, when he was in an odd-numbered rakah of his prayer, would not rise until he had sat up straight.
حدثنا مسدد حدثنا هشيم عن خالد عن أبي قلابة
أنه رأى النبي ﷺ إذا كان في وتر من صلاته لم ينهض حتى يستوي قاعدا
Bukhari narrated it, Adhan, chapter on one who sits upright at an odd point in his prayer and then rises, hadith no. 823, from the hadith of Hushaym, through this route.