Muslim

Hadith Commentary:
(1)
Although this hadith mentions a warning for the one who raises his head before the imam, it is prohibited to perform any action before the imam during the prayer.
There is another well-known narration on this subject in which the Messenger of Allah (sallallahu alayhi wa sallam) said:
“Whoever raises or lowers his head before the imam, his forehead is in the grasp of Satan.”
Hafiz Ibn Hajar (rahimahullah) also mentioned this in Fath al-Bari (2/237),
but this narration is not authentically attributed (marfu‘an) to the Prophet.
See:
(al-Mu‘jam al-Awsat: 5/385, Hadith: 7692)
According to the majority, the one who does so is sinful, but his prayer is still valid. However, the correct view is that if someone does this deliberately, his prayer is invalid. Thus, Abdullah ibn Umar (radi Allahu anhu) said that such a person’s prayer does not count at all. This is also the position of Imam Ahmad and others, because a prohibition (nahi) implies corruption (fasad).
If his prayer were valid, then there would be hope for reward and no fear of punishment.
(Fath al-Bari: 2/238)
Allamah Sindhi (rahimahullah) says that it is astonishing that those who consider the prayer of such a person valid do so. These scholars do not consider the prayer of one who prays ahead of the imam in position to be valid, but they consider the prayer of one who precedes the imam in actions to be valid, whereas following (iqtida) in prayer is to be done in the actions themselves. In this regard, if the prayer of one who moves ahead of the imam in position is invalid, then the prayer of one who precedes the imam in actions should all the more be invalid.
(Hashiyat al-Sindi: 1/128)
(2)
Some people interpret the warning mentioned in the hadith metaphorically, meaning that the foolishness and lack of understanding of such a person is being expressed in these words, because transformation (maskh) is not permissible in this ummah. Moreover, despite frequent violations of this ruling, no incident of actual transformation has been reported.
This position is not based on reality, because it is not correct to absolutely negate transformation (maskh) in this ummah.
A hadith narrated from Aisha (radi Allahu anha) states:
“In the last era of this ummah, there will be swallowing (khasf), transformation (maskh), and pelting (qadhf).”
As for the question of why such an incident has not occurred, this is merely Allah’s grace that such a person is not immediately punished. There are many punishments for which a person becomes deserving, but Allah the Exalted pardons him. Also, the hadith only indicates that such a person is deserving of this punishment.
It is also possible that Allah the Exalted has given respite to such people.
(‘Umdat al-Ahkam: 4/313)
(3)
Hafiz Ibn Hajar (rahimahullah) has written a very fine point at the end of this discussion: The reason for competing with the imam is a person’s haste, so such a person should reflect that no matter how much he hastens, in the end he will have to say the salam with the imam.
(Fath al-Bari: 2/238)

623. Commentary:
To be heedless of the important obligations (wajibat) of prayer is a sign of extreme ignorance and foolishness; it is in this sense that this warning has been given. Therefore, it is obligatory (wajib) for the follower (muqtadi) to remain behind his imam in every situation.

829. Commentary:

➊ That is, as a punishment, because his action is foolish like that of a donkey. The donkey is proverbial for foolishness. Or, if the form is made according to the action, then such a person's face should resemble that of a donkey, or he has been likened to a donkey.

➋ This hadith is to be understood as an emphasis (on the prohibition). When a person cannot finish the prayer before the imam, then what else is raising the head before the imam except foolishness? However, according to the apparent meaning, Allah Ta'ala can also make such a person's head like the head of a donkey. One should remain fearful of this warning.

Benefits and Issues:
(1)
From such a severe warning, it is evident that raising the head from prostration (sajdah) or bowing (ruku‘) before the imam is a very grave sin.

(2)
Generally, Allah Ta‘ala does not give this type of punishment for sins in this world, but it is possible that a person may be punished in this world itself—especially when he commits the sin out of stubbornness or arrogance.

(3)
Raising the head before the imam does not bring any benefit to the person. Through this haste, he cannot finish the prayer before the imam. Therefore, such a useless act is nothing but foolishness.

Benefit:
From this hadith, it is understood that one should follow the imam in prayer; neither should one go into prostration (sujud) or bowing (ruku') before the imam, nor should one raise one's head from prostration or bowing before the imam. It is also established that Satan always incites opposition to the Qur'an and hadith; therefore, we should oppose Satan.