It is a most amazing thing to see a man in his house, who, when he hears the call to prayer, stands up in fear (of his Lord) and prepares himself, then leaves his house for the prayer, wanting nothing other than to perform it. He might be leaving his house on a dark, rainy night, squelching through mud and splashing in water and soaking his clothes. If it is in summer, he risks (being stung by) scorpions or other venomous pests in the dark.
In addition, he might be ill or weak, but still he does not abandon the mosque. And he bears all these things because of his longing for prayer and his sincere love for it. Only, prayer could cause him to leave his house in such circumstances. But when the Imam (leader of prayer) begins the prayer, deceives him, and he competes with the Imam (by anticipating him) in bowing (ruku`), prostrating (sujud), straightening up and going down. This is the deception of Satan, who desires to invalidate his prayer. Then he leaves the mosque, and there is no prayer (accredited) to him.
Another strange thing is that all of the worshippers behind the Imam agree that it is not permissible for anyone to leave the prayer until the Imam does so, all of them wait until the Imam’s tasleem (end of prayer). In spite of this, however, all of them — except as Allah wills— anticipate the Imam in bowing, prostrating, straightening up and going down, due to the deception of Satan, causing them to perform prayer imperfectly, negligently. This is their lot in Islam and it has been narrated in the hadith: “There is no share in Islam for the one who abandons Salah.” (Saying of `Umar ibn Al-Khattab)
It is a pillar of Islam — once it is gone, there is no religion, no Islam. So remember Allah in all your affairs in general, and in your prayer in particular. Hold fast to it, and take care not to lose (the reward of) it nor to be careless in its performance or to anticipate the Imam in it and beware of Satan’s deception and his causing you to lose the reward of it, for it is the last thing in your religion; and whoever loses the last of his religion has lost all of it, so hold fast to the last thing in your religion. And direct the Imam — O’ slave of Allah!, to take care in his prayer, to take pains in it whenever he bows or prostrates, for I prayed with you one day and I was unable to make tasbeeh (i.e. to say, Subhana Rabbi al-Adheem or Subhana Rabbi al-`Ala) three times in bowing or prostration, and this was due to the haste of the Imam —he neither permitted himself, nor the worshippers to take their time, instead he rushed (through the Salah). So inform him that the Imam — if he prays well — will receive the reward of his prayer and the equivalent to the reward of those performing prayer behind him. But if he performs prayer badly, he will bear that sin and the sin of those who perform it behind him.
Quoted with slight modifications from Ahmad ibn Hanbal’s Treatise on Prayer.