I heard Abu Humayd al-Saidi speaking among ten of the companions of the Messenger of Allah ﷺ, including Abu Qatada. Abu Humayd said, "I am the most knowledgeable among you concerning the prayer of the Messenger of Allah ﷺ." He then described it, including the following, "He would bow and place his palms firmly on his knees, neither letting his head droop down nor raising it up, but keeping it level."
سمعت أبا حميد الساعدي في عشرة من أصحاب رسول الله ﷺ فيهم: أبو قتادة فقال أبو حميد: أنا أعلمكم لصلاة رسول الله ﷺ وفيه: ثم يركع ويضع راحتيه على ركبتيه معتمدا، لا يصب رأسه ولا يقنع معتدلا.
It was narrated by Abu Dawud (730), Tirmidhi (304), and Ibn Majah (1061), all through Abdul-Hameed bin Jafar, who said, "Muhammad bin Amr bin Ata narrated to us," with the same wording, in a long hadith. Its sourcing was covered earlier in the chapter on raising the hands, and there is some discussion about it. Nasai (1040) also narrated it through Abdul-Hameed in a shortened form, saying, "When the Prophetﷺ bowed in ruku, he kept his back level, neither lowering his head nor raising it, and he placed his hands on his knees."His statement "he did not lower his head" means he did not let it droop downward. In Tirmidhi's version it reads "he did not incline his head," from tasweeb, which means bending the head downward, and the two expressions carry the same meaning. As for his statement "nor did he raise it," it means he did not lift his head so that it would be higher than his back, taken from the expression "he raised his head," meaning he held it upright. Rather, his head was kept level, between the two extremes.