When does the imam say the takbir for bowing: before bowing, while bowing, or after bowing?

The takbir for rukoo' (bowing) begins when one bends for it and ends before reaching it, thus occurring between standing and bowing. This is indicated by the Prophetic Sunnah and the view of the majority of scholars. Some jurists have been strict, believing that if one completes the takbir before bending or after reaching rukoo', their prayer becomes invalid if done intentionally. However, if done out of forgetfulness, they must perform the prostration of forgetfulness (sujud as-sahw). The correct view is that this is excused to alleviate hardship. The takbir should accompany the movement, but if one reaches rukoo' before completing the takbir, there is no harm. The best is for the takbir to be between the two pillars (standing and bowing) as much as possible.