Dought & clear, - When should the one who is praying behind the imam bow? When the imam says takbeer, or straight after the takbeer, or when?.

When we are praying behind the imam, when should we start to bow?
Should we bow when we hear the imam say Allaahu akbar or during that
or after he has finished saying it?.
Praise be to Allaah.
What is prescribed for the one who is praying behind the imam is to
follow his imam, not to do anything before him or at the same time as
him, or to delay doing it. Rather he should do things straight after
his imam does them.
This is indicated by the report narrated by al-Bukhaari (378) and
Muslim (417), from the hadeeth of Anas ibn Maalik and Abu Hurayrah
(may Allaah be pleased with them), according to which the Messenger of
Allaah(peace and blessings of Allaah be upon him) said: "The imam is
only appointed to be followed, so when he says takbeer, then say
takbeer; when he bows, then bow (and do not bow until he bows); when
he saysSami'a Allaahu liman hamidah, then sayAllaahumma rabbana
laka'l-hamd. If he prays standing, then pray standing." The phrase in
brackets was added in the version narrated by Abu Dawood (603).
The words of the Prophet(peace and blessings of Allaah be upon him):
"when he bows, then bow and do not bow until he bows", and the same
with regard to prostration, indicate that the person who is praying
behind the imam should not start to move to the next part of the
prayer until after the imam has done it. So he should not bow until
the imam bows, and he should not prostrate until the imam prostrates.
This is stated clearly in the hadeeth of al-Bara' ibn 'Aazib (may
Allaah be pleased with him) who said: When the Messenger of Allaah (S)
said "Sami'a Allaahu liman hamidah," no one of us would bend his back
until the Messenger of Allaah (S) had gone down in prostration, then
we would go down in prostration after him. Narrated by al-Bukhaari
(811)and Muslim (474).
Al-Nawawi said inSharh Muslim: This hadeeth shows that this is one of
the etiquettes of prayer and it is Sunnah for the one who is praying
behind the imam not to start going down (for prostration) until the
imam has placed his forehead on the ground. End quote.
Based on this, what is prescribed for the one who is praying behind
the imam is to follow his imam straight after he has moved to the next
part of the prayer. So what counts is the action of the imam, not his
takbeer. This applies to those who can see the imam. As for the
members of the congregation who cannot see him, then they should
follow his words, and they should start to move to the next part of
the prayer after the imam finishes saying the takbeer.And Allaah knows
best.