When the Prophet (may Allah's peace and blessings be upon him) was performing an odd-numbered Rak‘ah, he would not get up until he had been settled in the sitting position
Authentic hadithعن مالك بن الحويرث رضي الله عنه : أنه رأى النبي صلى الله عليه وسلم يصلي، فإذا كان في وِتْرٍ من صلاته لم ينهض حتى يستوي قاعداً.
Mālik ibn al-Huwayrith (may Allah be pleased with him) reported: I saw the Prophet (may Allah's peace and blessings be upon him) pray and when he was performing an odd-numbered Rak‘ah, he would not get up until he had been settled in the sitting position.
Explanation
It was part of the Prophet’s Sunnah that when he stood from the first to the second Rak‘ah or from the third to the fourth one, he would briefly settle in the sitting position first, before standing up. This is called “the sitting of rest”, and it was reported by a group of the Companions, may Allah be pleased with them.
Source · Narrated by Bukhari
From the Hadith Encyclopedia (hadeethenc.com).