Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Mughirah bin Shu’bah (may Allah be pleased with him), he says that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the imam (forgets and instead of sitting for tashahhud) begins to stand up after two rak‘ahs, if he remembers before standing up straight, he should sit down; and if he stands up straight, then (he should continue with the third rak‘ah and) not sit for tashahhud, and (because the middle tashahhud was missed, before the salam) he should perform two prostrations."