It is permissible for the imam to begin the takbir while he is descending
Machine translationRead the Arabic original
The questioner : The imam, before he prostrates, says the takbir. The believer loves prostration; he says the takbir before prostrating.
The Shaykh : He says the takbir before he prostrates. If he says the takbir before he prostrates, his prostration is valid before it reaches the ground.
The questioner : Good, describe it for us.
The Shaykh : We say that this is not correct. We mean, there is nothing wrong with the imam beginning while he is descending, provided that he finishes when he reaches the prostration.
The questioner : A little, a little.
The Shaykh : Is this what you want? Yes.
The Shaykh : He says the takbir before he prostrates. If he says the takbir before he prostrates, his prostration is valid before it reaches the ground.
The questioner : Good, describe it for us.
The Shaykh : We say that this is not correct. We mean, there is nothing wrong with the imam beginning while he is descending, provided that he finishes when he reaches the prostration.
The questioner : A little, a little.
The Shaykh : Is this what you want? Yes.
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 1856يجوز للإمام أن يبدأ التكبير في أثناء هويه📔 الشرح الممتع على زاد المستقنع - ابن عثيمين — youtube.com
The Arabic text is a machine transcription of the audio recording.