The Book of Prayers
حَدَّثَنَا خَلَفُ بْنُ هِشَامٍ، وَأَبُو الرَّبِيعِ الزَّهْرَانِيُّ، وَقُتَيْبَةُ بْنُ سَعِيدٍ، كُلُّهُمْ عَنْ حَمَّادٍ، - قَالَ خَلَفٌ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، - عَنْ مُحَمَّدِ بْنِ زِيَادٍ، حَدَّثَنَا أَبُو هُرَيْرَةَ، قَالَ قَالَ مُحَمَّدٌ صلى الله عليه وسلم " أَمَا يَخْشَى الَّذِي يَرْفَعُ رَأْسَهُ قَبْلَ الإِمَامِ أَنْ يُحَوِّلَ اللَّهُ رَأْسَهُ رَأْسَ حِمَارٍ " .Abu Huraira reported:The Messenger of Allah (ﷺ) said: Does the man who lifts his head ahead of the Imam (from prostration) not fear that Allah may change his head into the head of an ass?
From Sahih Muslim, whose reports the scholars accept as authentic.