(10) The Book of Leading the Prayer (Al-Imamah)
كتاب الإمامة
(38) Chapter: Preceding the Imam
(38) باب مُبَادَرَةِ الإِمَامِ
Sunan an-Nasa'i 828 
It was narrated that Abu Hurairah said:
"Muhammad (ﷺ) said: 'Does the one who raises his head before the Imam not fear that Allah may turn his head into the head of a donkey?"'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ مُحَمَّدِ بْنِ زِيَادٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ مُحَمَّدٌ صلى الله عليه وسلم ‏ "‏ أَلاَ يَخْشَى الَّذِي يَرْفَعُ رَأْسَهُ قَبْلَ الإِمَامِ أَنْ يُحَوِّلَ اللَّهُ رَأْسَهُ رَأْسَ حِمَارٍ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 828
In-book reference : Book 10, Hadith 52
English translation : Vol. 1, Book 10, Hadith 829