The Eid Al-Adha prayer First then the sacrifice..

Narrated Al-Bara’: 

I heard the Prophet (s.a.w) 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).” 

Bukhari  (Book #15, Hadith#71)