Mishkat al-Masabih 4 Prayer كتاب الصلاة 558

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَمَا يَخْشَى الَّذِي يَرْفَعُ رَأْسَهُ قَبْلَ الْإِمَامِ أَنْ يُحَوِّلَ اللَّهُ رَأْسَهُ رَأْسَ حمَار»

Abu Huraira reported God’s Messenger as saying, “Does he who raises his head before the imam not fear that God may change his head into a donkey’s?” (Bukhari and Muslim.)