What must a worshipper do if they realize during the second unit (rak'ah) of prayer that they only performed one prostration in the first unit with the Imam, or are unsure if they performed one or two prostrations in the first unit?
General Chapter
Al-Mughni
Book of Friday Prayer
Primary text
If the worshipper has not yet begun the recitation for the second unit, they must return, perform the missing prostration for the first unit, complete it, and then perform the second unit to complete the congregational prayer (Jumu'ah). This ruling is explicitly stated by Ahmad, as narrated by Al-Athram. In either case, the prayer is completed as Jumu'ah, according to the narration of Al-Athram.
Supporting text
If the worshipper has already begun the recitation of the second unit when realizing the deficiency in the first, the first unit is nullified, and the second unit becomes the first unit. Another narration suggests that in a crowded situation, the deficiency should result in completing the prayer as four units of Dhuhr prayer because a complete unit was not attained with the Imam.