I prayed behind the Messenger of Allah ﷺ, and we asked him about the length of his bowing and prostration. He said, "It is the length of time it takes a person to say, 'Glory be to Allah and praise be to Him,' three times."
حدثنا محمد بن عبد الرحمن الطفاوي حدثنا سعيد الجريري
صليت خلف رسول الله ﷺ فسألناه عن قدر ركوعه وسجوده فقال قدر ما يقول الرجل سبحان الله وبحمده ثلاثا
Its isnad is daif, due to the anonymity of the narrator from the companion. The measure of ruku and sujud similar to this was narrated by Abu Dawud 1/209, no. 870, in the book of Prayer, chapter on what a man says in ruku, from a man from Uqba bin Amir, and Tirmidhi 2/47, no. 261, in the book of Prayer, chapter on what has been said about glorification in ruku, from Ibn Masud, and he said, it is not connected. And Ibn Majah 1/287, no. 888, from Huzaifa bin Yaman, and Bayhaqi 2/111, similar to Ahmad.Tirmidhi said, the practice among the people of knowledge is upon this, meaning that this is the minimum obligation. He then quoted from Ibn Mubarak that he said it is recommended to increase it up to five. It has already been mentioned that Umar bin Abdul-Aziz used to increase it up to ten, and this is the practice of the people of knowledge. As for the people of ignorance, they increase it nowadays up to thirty or forty, driving people away.