Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.086A.

If the imam forgets a verse, it is permissible for a follower to remind him of it, regardless of whether the recitation is a part of the obligatory recitation or not. Ibn ‘Umar reports that the Messenger of Allah prayed and had some confusion in his recitation. When he finished, he said to ‘Umar: “Were you present with us [during the prayer]?” He replied: “Yes.” So, the Prophet sallallahu alehi wasallam asked him: “What prevented you from correcting me?” This is related by Abu Dawud and others and its narrators are trustworthy.