Language:
كتاب العيدين
13  
The Two Festivals (Eids)
(3)
Chapter: The legal way of the celebrations on the two 'Eid festivals
(3)
باب سُنَّةِ الْعِيدَيْنِ لأَهْلِ الإِسْلاَمِ
Sahih al-Bukhari 951

Narrated Al-Bara':

I heard the Prophet (p.b.u.h) delivering a Khutba saying, "The first thing to be done on this day (first day of `Id ul Adha) is to pray; and after returning from the prayer we slaughter our sacrifices (in the name of Allah) and whoever does so, he acted according to our Sunna (traditions)."

حَدَّثَنَا حَجَّاجٌ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ أَخْبَرَنِي زُبَيْدٌ، قَالَ سَمِعْتُ الشَّعْبِيَّ، عَنِ الْبَرَاءِ، قَالَ سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَخْطُبُ فَقَالَ ‏ "‏ إِنَّ أَوَّلَ مَا نَبْدَأُ مِنْ يَوْمِنَا هَذَا أَنْ نُصَلِّيَ، ثُمَّ نَرْجِعَ فَنَنْحَرَ، فَمَنْ فَعَلَ فَقَدْ أَصَابَ سُنَّتَنَا ‏"‏‏.‏
Reference : Sahih al-Bukhari 951
In-book reference : Book 13, Hadith 3
USC-MSA web (English) reference : Vol. 2, Book 15, Hadith 71
  (deprecated numbering scheme)