Is it Haram to forget the Qur’an? I once heard that there was a Hadith about forgetting it. If there is, can you send me a copy preferably in Arabic?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Forgetting the Qur'an is forbidden.  There is a consensus of scholars over this issue.  The Shafi’e scholars even consider that a major sin.  The evidence for this is the Hadith reported by Al-Tirmizi from Anas Ibn Malik that the Prophet (Sallallahu Alaihi wa Sallam) said: “The sins of my Ummah were exposed in front of me and I did not see any sin greater than the sin of a person who memorized a Surah of the Qur'an then forgets it."

Allah knows best.