Is the prayer of one who said the opening Takbir, then went down for Ruku' (bowing), and the Imam rose before his Ruku', so the worshipper straightened up before the Imam and prostrated with him, valid?

The rak'ah is valid if the congregant bows before the Imam straightens up, even if the Imam rises before the congregant achieves a state of tranquility (طمأنينة). However, if the Imam raises his head before the congregant bows, that rak'ah is not counted. It is better for the congregant to rise in accordance with the Imam, and his prayer is valid, as his following the Imam in standing for the bowing or preceding him in it does not harm it. One must turn away from obsessive doubts in this regard.