Reference: Sharh ‘Umdatil-Fiqh – Tape No.10, Question No.3
Question: Some people who are capable of standing [during the prayer, choose instead to] sit during the second rak’ah of the Fajr prayer until the imaam is about to bow [into rukoo’], so he bows along with the imaam. So is his prayer correct?
Response: No, he has left a pillar of the prayer [which necessitates] that he stand. At a minimum, it is imperative that he stand for the duration [of the imaam’s recital] of [soorah] al-Faatihah, and if he stands for any less than the duration [of the imaam’s recital] of [soorah] al-Faatihah without a genuine reason, then his prayer is not correct because he has left a pillar [of the prayer]; so he must be aware of this.