Does Missing a Wajib Behind the Imam Invalidate the Muqtadi’s Salah?

No, if a wajib is missed behind the imam, the muqtadi’s (follower’s) salah does not automatically become invalid. The validity of the prayer depends on the circumstances and whether Sajdah Sahw is required.

In congregational prayer, the muqtadi follows the imam. If the imam unintentionally misses a wajib act, the imam should perform Sajdah Sahw, and the followers should also follow him.

Missing a wajib does not immediately invalidate the prayer. If the omission happened by mistake and Sajdah Sahw is performed correctly, the salah remains valid.

However, if a wajib is deliberately left without a valid reason, different rulings may apply according to the situation.

Fiqh Reference

Al-Hidayah – Kitab al-Salah
Radd al-Muhtar – Kitab al-Salah

Conclusion

A muqtadi’s salah does not become invalid merely because a wajib was missed behind the imam. The ruling depends on the nature of the omission and whether the required correction, such as Sajdah Sahw, was performed.