Sahih Muslim
The Book of Sacrifices (Kitab Al-Adahi)
Book Number: 022 / Hadith Number: 4834
Anas b. Malik reported that Allah's Messenger (may peace be upon him) offered the 'Id prayer and then delivered the sermon giving the command: He who slaughtered the animal before prayer should slaughter (another animal as a sacrifice). The rest of the hadith is the same.