Sahih Muslim Book 022, Hadith Number 4826.
Chapter : The proper time for sacrifice.
Al-Bara' reported Allah's Messenger (may peace be upon him) having said: He who observes prayer like our prayer and turns his face towards our Qibla (in prayer) and who offers sacrifices (of animals) as we do, he must not slaughter the (animal as a sacrifice) until he has completed the prayer. Thereupon my maternal uncle said: Messenger of Allah, I have sacrificed the animal on behalf of my son. The Messenger of Allah (may peace be upon him) said: This is the thing in which you have made haste for your family. He said: I have a goat with me better than two goats. Thereupon he said: Sacrifice it for that is the best.