Abu Hurairah (may Allah be pleased with him) narrates that Muhammad (peace be upon him) or Abu al-Qasim (peace be upon him) said: "Does the person who raises his head before the Imam not fear that Allah may turn his head into the head of a donkey?"
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1600