Is congregational prayer valid if the imam inadvertently adds a fifth rak'ah and does not return after being alerted by the worshippers?

Scholars differed regarding the ruling on the prayer of an Imam who mistakenly added an extra unit (rak'ah):

Hanbalis: They hold that the ignorant person is like someone who forgets, and they adduced as proof that error and forgetfulness are pardoned; hence, his prayer is valid. Malikis: They hold that the ignorant person is like someone who acts intentionally, because ignorance of obligatory rulings is a sin; hence, his prayer is invalid.

As for the congregants (ma'mumun) who followed the Imam in the addition, believing his action to be correct, their prayer is valid. It is recommended for the Imam to repeat his prayer to avoid scholarly disagreement.