Is it permissible for the Imam of the congregation to prolong the supplication after the prayer?

Sheikh al-Islam Ibn Taymiyyah opined that congregational supplication (du'a) after the obligatory prayer is not legislated and there is no evidence for it. The Prophet, peace and blessings be upon him, used to supplicate at the end of his prayer before concluding it. It has not been reported that he and the congregants all supplicated together after exiting the prayer. Rather, he would turn to his companions, remember Allah, and teach them the remembrance after the prayer.