Should I begin to move to follow the Imam at the beginning or end of the words "Allahu Akbar" and "Sami'a Allahu liman Hamidah"?

Following the Imam in the sayings of the prayer means lagging behind him. For the opening takbir and the taslim, the ma'mum (follower) should not begin until the Imam has finished. In other sayings, it is preferable to lag behind. The ma'mum's prayer is not invalidated by performing them simultaneously, except for the taslim, according to some scholars.