Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(53)
Chapter: The sin of the one who raises his head before the Imam (raises his head)
(53)
باب إِثْمِ مَنْ رَفَعَ رَأْسَهُ قَبْلَ الإِمَامِ
Sahih al-Bukhari 691

Narrated Abu Huraira:

The Prophet (ﷺ) said, "Isn't he who raises his head before the Imam afraid that Allah may transform his head into that of a donkey or his figure (face) into that of a donkey?"

حَدَّثَنَا حَجَّاجُ بْنُ مِنْهَالٍ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ مُحَمَّدِ بْنِ زِيَادٍ، سَمِعْتُ أَبَا هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ أَمَا يَخْشَى أَحَدُكُمْ ـ أَوْ لاَ يَخْشَى أَحَدُكُمْ ـ إِذَا رَفَعَ رَأْسَهُ قَبْلَ الإِمَامِ أَنْ يَجْعَلَ اللَّهُ رَأْسَهُ رَأْسَ حِمَارٍ أَوْ يَجْعَلَ اللَّهُ صُورَتَهُ صُورَةَ حِمَارٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 691
In-book reference : Book 10, Hadith 86
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 660
  (deprecated numbering scheme)