When must the follower (*Ma'mum*) say the initial Takbir (*Takbiratul Ihram*) relative to the Imam?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
The follower must not utter the initial Takbir until the Imam has completely finished uttering his Takbir. The evidence for this is the statement of the Prophet, peace be upon him: 'The Imam is appointed only to be followed; therefore, when he says Takbir, say Takbir.' This hadith is agreed upon (Muttafaq 'alayh). The ruling regarding Ruku' (bowing) is analogous in that the follower follows the Imam, but performing Ruku' simultaneously with the Imam does not invalidate the prayer because the follower has already entered into the prayer state, whereas here (regarding the initial Takbir), the situation is different.
Supporting text
Abu Hanifa holds the opinion that the follower should say the Takbir concurrently with the Imam, similar to how he performs Ruku' with the Imam. If a follower says Takbir before the Imam, that Takbir is invalid, and the follower must repeat the Takbir after the Imam has completed his own.