All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad (sallallaahu 'alayhi wa sallam) is His slave and Messenger.
It is from the Sunnah that the worshipper begins saying the takbir of transition, or the tasmi’ at the very start of entering into the pillar of the prayer. Thus, he should begin saying the takbir when he starts to bend down for ruku’, and begin saying the tasmi’ when he starts to rise from ruku’.
It is stated in Al-Mughni byIbn Qudamah
: “it is recommended that one begins saying the takbir at the start of raising his head from prostration, and completes it upon standing upright, so that the takbir encompasses the entire pillar during which it is legislated. The same applies to the rest of the takbirs...(etc.).”
Based on the foregoing, if the mentioned Imam pronounces the takbir for ruku’ while still standing, or utter the tasmi’ after having fully stood upright, he has deviated from the sunnah; however, his prayer is valid, as is the prayer of those following him.
Allah Knows best