It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him); he said: My maternal uncle, my mother's brother, told me that the Messenger of Allah (peace and blessings be upon him) said: "The first act of worship of this day, meaning the Day of Sacrifice, is the prayer, then the sacrifice after the prayer." So I walked to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, I slaughtered before I prayed, and I have a fine one-year-old young goat that is dearer to me than the meat of two goats; shall I slaughter it? He (peace and blessings be upon him) said: "Yes, but it will not suffice for anyone after you."
Hadith Referenceالمعجم الكبير للطبراني / باب الهاء / 18724