He prayed an extra rak’ah in Tarawih

The imaam stood up for a third rak’ah during Tarawih prayers and he finished the prayer, then they told him that he had prayed three rak’ahs. Should he prostrate sajdat al-sahw (prostration of forgetfulness) or complete four rak’ahs?

Praise be to Allah, and blessings and peace be upon the Messenger of Allah:

We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allah preserve him, who answered as follows:

He should do sajdat al-sahw, then complete the prayer two by two, just as he should do if it were an obligatory prayer.

And Allah knows best.