If someone gives the closing salam before completing the required rakaat and only later realises it, may the prayer be continued or must it be repeated from the start?

Summary

An obligatory prayer ended by salam before completing its rakaat may be continued by performing the missing rakaat, based on the hadith of Dzul Yadain in Sahih Muslim, where the Prophet also performed the prostration of forgetfulness. This holds provided there was no excessive speaking or movement after the mistaken salam. If a long time has passed with much movement, such as bathing or eating and drinking, the prayer must be performed again from the beginning.

Written in our own words from the original fatwa — check the source before acting on it.