Is it obligatory to follow an imam who does not observe the Sunnah of pausing between the two prostrations, or is it permissible for the one praying behind him to differ from him and pause for a moment?

Sitting between the two prostrations is an obligatory pillar of prayer, not a sunnah, based on the hadith of the one who prayed incorrectly and Aisha's (may Allah be pleased with her) statement: "When he (the Prophet) raised his head from prostration, he would not prostrate again until he had sat up straight." The prayer becomes invalid if it is omitted, and it is not covered by the command to follow the Imam. The Imam who omits it must be advised, and the prayer is not valid if one follows him in omitting it.