Is it permissible to read from the mushaf behind the Imaam in prayer?

It is not permissible. You must be listening attentively; you should never follow him reading. It is only upon you to listen to the recitation and not to read along. Do not follow along with him in the Mushaf, and do not read along with him during his recitation. All of that is in opposition to the teachings of Islam—rather, it's feared that your prayer may not be correct. Because it's a must that you be attentive, mandatorily. You must listen, and follow along with the Imaam by listening attentively.