Is the prayer valid if an Imam leading travelers forgets and completes the prayer fully (four rak'ahs)?
Chapter on the Traveler's Prayer
Al-Mughni
Book of Prayer
Primary text
If an Imam leading travelers intentionally completes the prayer fully, the prayer of the Imam and the followers is valid. Prostration of forgetfulness (sujud as-sahw) is not obligatory for the Imam because this addition, which is not invalidating if done intentionally, does not necessitate prostration for forgetfulness, similar to additions in speech acts, such as reciting during prostration or sitting. The preferred view is that no prostration is needed because the fundamental action was performed and requires no rectification.
Supporting text
There is a difference of opinion regarding whether prostration of forgetfulness is legislated for such an addition; it depends on the two established narrations concerning the ruling on adding such verbal recitations. The legality of prostration arises from the view that this addition diminishes virtue and compromises perfection, thus resembling reciting in the wrong place or reciting a Surah in the last two rak'ahs.