Does the imam's saying, "Prostration, brothers," during the prayer invalidate it?

What the Imam did is a clear mistake, for speaking during prayer invalidates it if done knowingly and intentionally. If the Imam was ignorant of the ruling, then his prayer and the prayer of those behind him are valid according to the preponderant opinion, which is the madhhab of Malik and Shafi'i, and a narration from Ahmad. However, if he was aware of the ruling, then his prayer is invalid, and he must repent.

Imams must strive to understand the affairs of their religion. Scholars have agreed that intentional speech during prayer invalidates it, but they differed on the speech of one who forgets or is ignorant. Some equated the forgetful, the intentional, and the ignorant in invalidating the prayer, while others differentiated between the speech of the forgetful and the ignorant and the speech of the intentional, citing the hadith of Dhul-Yadain and the hadith of Mu'awiyah ibn al-Hakam.