Sahih al-Bukhari, Hadith 976

The Prophet (ﷺ) went towards Al-Baqi (the graveyard at Medina) on the day of Id-ul-Adha and offered a two-rak`at prayer (of `Id-ul-Adha) and then faced us and said, "On this day of ours, our first act of worship is the offering of prayer and then we will return and slaughter the sacrifice, and whoever does this concords with our Sunna; and whoever slaughtered his sacrifice before that (i.e. before the prayer) then that was a thing which he prepared earlier for his family and it would not be considered as a Nusuk (sacrifice.)" A man stood up and said, "O, Allah's Messenger (ﷺ)! I slaughtered (the animal before the prayer) but I have a young she-goat which is better than an older sheep." The Prophet (p.b.u.h) said to him, "Slaughter it. But a similar sacrifice will not be sufficient for anybody else after you."