وَنِيَّةُ الْمُتَابَعَةِ لِلْمُقْتَدِي
A follower needs the intention of following the Imam in addition to the prayer intention. The follower's Tahrimah is not completed before the Imam's Tahrimah; entering after the Imam has begun is valid when the other conditions are met.
The practical rule is to wait for the Imam's opening Takbir and then say one's own Takbir, avoiding the doubtful case of finishing before the Imam.
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Prayer Conditions: Time, Intention, and the Opening Tahrimah
وَنِيَّةُ الْمُتَابَعَةِ لِلْمُقْتَدِي
A follower needs the intention of following the Imam in addition to the prayer intention. The follower's Tahrimah is not completed before the Imam's Tahrimah; entering after the Imam has begun is valid when the other conditions are met.
The practical rule is to wait for the Imam's opening Takbir and then say one's own Takbir, avoiding the doubtful case of finishing before the Imam.