When the imam has reached the position he is moving to, regardless of his takbeer. But if you can’t see him, then only move after he finishes his Takbeer.

“The person who is praying behind an imam should not move to the next posture of the prayer until the imam has reached it”. Some imams say the takbeer after they have reached the next position, some say it before they have reached. When should I start moving towards the next position? With the takbeer or after the takbeer?

Prayer | May 20, 2015 5:54 pm
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During prayer I made low mmm sound while thinking about something for a second. I completed my prayer because I have bad waswas that always makes me think my prayer is invalid. But is my prayer valid if I made this sound?
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If the Imam is in the last Tashahhud, how should a person join him?  Should he just sit down or should he do takbiratul Ihram and folding the hands on chest, then sit down?