At the opening takbir the follower intends both the prayer and following the designated imam. The mental intention—not repeating a prescribed Arabic or Urdu formula—is relevant. Later conversion from solitary prayer to following has its own detailed rules.
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When Intention to Follow Must Be Present
Prayer Recitation: Surah Selection, Minimum Amount and Following the Imam
At the opening takbir the follower intends both the prayer and following the designated imam. The mental intention—not repeating a prescribed Arabic or Urdu formula—is relevant. Later conversion from solitary prayer to following has its own detailed rules.