I pledged allegiance to the Messenger of Allah ﷺ that I would not bow down except while standing.
أخبرنا إسمعيل بن مسعود قال حدثنا خالد عن شعبة عن أبي بشر قال سمعت يوسف وهو ابن ماهك يحدث
بايعت رسول الله ﷺ أن لا أخر إلا قائما
No references are available.
[Its chain of transmission is sahih] Ahmad narrated it, 3/402, from the hadith of Shuba, through him, and it is also in Kubra, no. 671. Hakeem is Ibn Hizam.