All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Shaffii scholars are of the view that it is disliked to read from the Mus-haf while praying since reading causes the Imam to move a little. However, his prayer is valid Insha`Allah (God willing).
"Even if he read from the Mus-haf and turned its pages, his prayer isn`t invalidated because he made a little movement, it wasn`t continuous, it didn`t look that he was not engaged in prayer, and whatever invalidates in larger quantity is considered disliked, even in smaller quantity." [Asna al-Matalib, vol.1/pp.183]. This is also the view of the Maliki scholars as clarified in their book: " [Jawahir al-Ikleel, vol.1/pp.74].
We recommend that you (asker) recite from what is stable in your memory, but if it happened once that you forgot then there is no harm in that particularly since it is recommended that an Imam doesn`t prolong congregational prayers. On our part, we prefer that you don`t recite from the Mus-haf because Hanafii scholars consider it spoon feeding which invalidates prayer.
As regards Taraweeh (Night prayer in Ramadan), you are allowed to recite from the Mus-haf because there is a need to do so and this need removes the reprehensibility. And Allah knows best.