A rak'ah is caught by reaching the Ruku while the prayer leader is still in Ruku.
Merely:
• saying the opening Takbir,
• then remaining standing,
• while the prayer leader rises from Ruku
does not count as catching that rak'ah.
The follower must actually enter Ruku before the prayer leader rises from it.
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Fajr Sunnah, Joining the Congregation, and Catching a Rak'ah
A rak'ah is caught by reaching the Ruku while the prayer leader is still in Ruku.
Merely:
• saying the opening Takbir,
• then remaining standing,
• while the prayer leader rises from Ruku
does not count as catching that rak'ah.
The follower must actually enter Ruku before the prayer leader rises from it.