What should one do who sits for the final Tashahhud mistakenly in the third rak'ah instead of the fourth in Asr prayer, and should he complete the prayer then perform the prostration of forgetfulness (sujud al-sahw) before the taslim (final salutation)?

The prayer must be completed by standing for the fourth رکعة (unit of prayer) and completing it, then sitting for the final Tashahhud (definition) (testification of faith) in its proper place. It is recommended to perform the prostrations of forgetfulness (sujood as-sahw) due to sitting and reciting the Tashahhud after the third رکعة. The prostrations of forgetfulness can be performed either before or after the salutation (taslim (definition)).