The Messenger of Allah ﷺ used to pray both standing and sitting. When he began the prayer standing, he would perform the bowing while standing, and when he began the prayer sitting, he would perform the bowing while sitting.
حدثنا وكيع قال حدثنا يزيد يعني ابن إبراهيم عن ابن سيرين عن عبد الله بن شقيق
كان رسول الله ﷺ يصلي قائما وقاعدا فإذا افتتح الصلاة قائما ركع قائما وإذا افتتح الصلاة قاعدا ركع قاعدا
Its isnad is sahih. Yazid bin Ibrahim is al-Tustari, and he is trustworthy, whose narrations are established and accepted by the community of scholars. The hadith was recorded by Nasai, 2/219, no. 1647, in the chapter on Qiyam al-Layl. A similar version is found in Muslim and others, as mentioned earlier.