Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
The worshiper may neither anticipate the imam nor move in sync with him. All of the follower’s movements and pronouncements of takbîr should take place only after the imam completes his actions.
Al-Barâ’ said: “When the Prophet (peace be upon him) used to say: ‘Sami` Allahu liman hamidah’ we would remain standing until we saw him place his forehead on the ground. Then we would follow him.” [Sahîh al-Bukhârî and Sahîh Muslim]
In another narration, it reads: “None of us would bend his back until the Prophet (peace be upon him) went into in prostration. Then we would go into prostration after him.” [Sahîh al-Bukhârî]
Abû Mûsâ relates that the Prophet (peace be upon him) said about prayer: “If he bows, then you should bow, because the imam is to bow before you do so and he is to rise up before you do so.” [Sahîh Muslim (404)]
And Allah knows best.
Source: Islam Today