All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The people led in prayer by an Imaam should follow him when bowing, prostrating, and in all other movements of the prayer, and they are forbidden from preceding him, as the Prophet said: "The Imaam of the congregation is there only to be followed." [Al-Bukhari and Muslim] The Prophet also said: "Do not precede me when bowing, or prostrating, or standing up or when finishing the prayer." [Muslim] Another Prophetic narration reads: "Does not one of you fear that if he raises his head before the Imaam, Allaah will turn his head into the head of a donkey, or make his image like that of a donkey." [Al-Bukhari and Muslim] For more benefit, please refer to Fatwa 85130.
As regards the sitting for rest (between the two prostrations), the people led in prayer should follow the Imaam if he observes it, for the general meaning of the Prophetic narration: "The Imaam of the congregation is there only to be followed." However, if the congregation did not observe the sitting for rest like the Imaam, there is no harm on them as the sitting for rest is only recommended and if one fails to observe it, it is not considered an aberrant error, as Ibn Hajar Al-Haytami said. Nonetheless, if the congregation do not follow the Imaam, and he precedes them when standing up [i.e. they delay in following him], then they are wrong but their prayer is valid.
With regard to what is best, what the Imaam did [observing the sitting for rest], or what the congregation did [in not observing it], then the sitting for rest is a confirmed Sunnah of the Prophet and whoever does it imitating the Prophet he will be rewarded.
For more details on the sitting for rest, please refer to Fatwa 86323.
Allaah Knows best.