Dought & clear, - When should the takbeeraat of movementbe said during the prayer?.

When the imam is praying, when should he say takbeer for bowing, for
example? Should he say takbeer before he bows, whilst bowing or after
bowing?.
Praise be to Allaah.
What is prescribed for everyone who prays (the imam, the one who is
praying behind the imam and the one who is praying alone) is for the
takbeer for bowing to accompany the movement. So he should start
saying takbeer when he starts to bow, and end it before he reaches the
bowing position, so that his takbeer comes between the two postures of
standing and bowing.
The Sunnah indicates that the takbeer should accompany the intended
movement such as bowing, prostrating and standing up. It is narrated
inal-Saheehaynthat Abu Hurayrah (may Allaah have mercy on him) said:
When the Messenger of Allaah (S) stood up to pray, he would say
takbeer when he stood up, then he would say takbeer when he bowed,
then he would say "Sami'a Allaahu limanhamidah(Allaah hears those who
praise Him)" until he was standing up straight after bowing, and when
he was standing he would say, "Rabbana wa laka'l-hamd(our Lord, to You
be praise)." Then he would say takbeer when he went down in
prostration, then he would say takbeer when he raised his head, then
he would say takbeer when he prostrated, then he would say takbeer
when he raised his head, and he did that throughout the prayer until
he finished. And he would say takbeer when he stood up after two
rak'ahs, after sitting."
Narrated by al-Bukhaari (789) and Muslim (392).
This hadeeth shows that the takbeer for bowing should be said whilst
bending down to bow, and the takbeer for prostration should be said
when going down for prostration, and the takbeer for standing up from
prostration should be said whilst rising, and so on. This was stated
by al-Nawawi inSharh Muslim, and he said that it is the view of the
majority of scholars.
Some of the fuqaha' were very strict on this point, and said that if
the worshipper starts to say the takbeer when he is standing, before
he bends down, or he completes it after he reaches the bowing posture,
that does not count and he has failed to say takbeer properly, because
he has not done it at the right time. According to the view that
takbeer is obligatory, he has invalidated his prayer if he did that
deliberately, and if he did it by mistake he must perform the
prostration of forgetfulness (sujood al-sahw). The correct view,
however, is that this is forgivable, so as to avoid undue hardship.
Al-Mardaawi said inal-Insaaf(2/59): Majd and others said: The takbeer
for going down and rising and standing up should start when the
movement begins and end when it ends. If he completes it partway
through, that is sufficient [i.e., if it happens between the two
postures without elongating it], because it has not been done at the
wrong time. There is no scholarly difference of opinion on this
matter.
But if he starts it before that or ends it after that, and some of it
happens outside the proper place, then this is like not doing it at
all, because he did not complete it at the right time. It is like one
who completes his recitation whilst bowing, or who starts to recite
the tashahhud before sitting.
But he may be forgiven for that, because avoiding it is difficult, and
mistakes are often made, and it is too harsh to suggest that this
invalidates the prayer or requires the prostration of forgetfulness.
End quote.
Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) said: The fuqaha'
(may Allaah have mercy on him) said: If he starts to say the takbeer
before he bends down, or he completes it after reaching the bowing
posture, then that is not valid, because they said that this takbeer
is for movement and its place is between the two postures. If it is
included in the first posture it is not valid and if he includes it in
the second posture it is not valid, because that is a place where this
dhikr is not prescribed. It is not prescribed to say takbeer whilst
standing or whilst bowing, rather the takbeer comes in between
standing and bowing.
Undoubtedly this view is valid to some extent, because the takbeer is
the sign of movement, so it should be done whilst moving.
But the view that his prayer is invalidated if he completes it after
he reaches the bowing position or he starts it before he starts to
bend down causes hardship for people, because if you think about how
people are nowadays, many people do not know this, and some of them
say takbeer before they start to bend down, and some of them reach the
bowing position before they finish saying it.
It is strange that some ignorant imams have come up with a mistaken
ijtihad and say "I will not say takbeer until I reach the bowing
position, because if I say takbeer before I reach the bowing position,
the people praying behind me will bow before I do, and they will bend
down before I reach the bowing position, and perhaps they will reach
it before I do." This is a strange kind of ijtihad, whereby you would
invalidate your own worship, according to the view of some scholars,
in order to ensure the validity of the worship of someone else who is
not enjoined to do things before you, rather he is enjoined to follow
you.
Hence we say: this ijtihad is inappropriate, and we call the one who
comes up with this kind of ijtihad ignorant … because he is ignorant
and he does not know that he is ignorant.
If, let's say, you say takbeer when you bend down, and you want to
finish it before you reach the bowing position, but you reach the
bowing position before you finish it, there is nothing wrong with
that.
The correct view is that if he starts the takbeer before bending down
to bow, and finishes it after that, there is nothing wrong with that.
If he starts it when he is bending down and ends it after he reaches
the bowing position, there is nothing wrong with that. But it is
better for it to be done between the two postures as much as possible.
The same applies during the saying of "Sami'a Allaahu liman hamidah"
and all the takbeeraat of movement. But if he does not start until
after he reaches the position that comes next, then this is not
valid. End quote fromal-Sharh al-Mumti'.
And Allaah knows best.