Sahih Ibn Khuzaymah

And indeed, the act of "tatbeeq" (placing the hands between the knees during bowing) is prohibited. It is not permissible that both "tatbeeq" and placing the hands on the knees are correct acts, just as we have mentioned (different) hadiths regarding the recitation of the Prophet ﷺ in prayers, and have mentioned the differences among the Companions regarding those surahs which the Prophet ﷺ used to recite in prayer, or as we have mentioned the differences among the Companions regarding the number of times the Prophet ﷺ washed the limbs during ablution. All these methods are permissible, but the act of "tatbeeq" in bowing has been abrogated and prohibited, and the Sunnah method is to place both hands on the knees.

Hazrat Mus'ab bin Sa'd narrates that when I used to perform ruku', I would place both my hands between my knees. My respected father, Sayyiduna Sa'd (may Allah be pleased with him), saw me doing this and forbade me, and said: Indeed, we used to do the same, then we were forbidden from it, then we were commanded to raise them towards our knees (i.e., to place them on the knees).

It is narrated from Sayyiduna Rifa’ah bin Rafi’ (may Allah be pleased with him) that a man entered the mosque and performed prayer, then the complete hadith is mentioned. He says that the Noble Prophet (peace and blessings of Allah be upon him) said: "Then when you bow (in ruku’), place your hands firmly on your knees until your bones become settled (i.e., you are at ease) in ruku’."