Because Allah, the Exalted, has said (in Surah Al-Baqarah): "O believers! Eat from the good things We have provided for you and give thanks to Allah if it is Him you worship. He has only forbidden you carrion, blood, the flesh of swine, and that which has been dedicated to other than Allah. But whoever is compelled by necessity, neither desiring it nor transgressing, there is no sin upon him." And Allah has said in Surah Al-Ma'idah: "But whoever is forced by severe hunger, with no inclination to sin..." And in Surah Al-An'am He said: "So eat of that upon which Allah's name has been mentioned, if you are believers in His verses. And what is [wrong] with you that you do not eat of that upon which Allah's name has been mentioned, while He has explained in detail to you what He has forbidden you, excepting that to which you are compelled. And indeed, many lead [others] astray through their [own] inclinations without knowledge. Indeed, your Lord—He is most knowing of the transgressors." And Allah has said in Surah Al-An'am: "Say, [O Prophet] , 'I do not find within that which was revealed to me [anything] forbidden to one who would eat it unless it be a dead animal or blood spilled out or the flesh of swine—for indeed, it is impure—or it be [that slaughtered in] disobedience, dedicated to other than Allah. But whoever is forced [by necessity] , neither desiring it nor transgressing, then indeed, your Lord is Forgiving and Merciful.'" Ibn Abbas, may Allah be pleased with them both, said the meaning of «مسفوحا» is "flowing blood." And in Surah An-Nahl He said: "So eat of the lawful and good things which Allah has provided for you, and be grateful for the favor of Allah, if it is Him you worship. He has only forbidden you carrion, flowing blood, the flesh of swine, and that which has been dedicated to other than Allah. But whoever is compelled by necessity, neither desiring it nor transgressing, then indeed, Allah is Forgiving and Merciful."

And Ibn Umar (may Allah be pleased with them both) said: This is Sunnah and this matter is well-known.

Narrated Al-Bara: The Prophet said (on the day of Idal-Adha), "The first thing we will do on this day of ours, is to offer the (`Id) prayer and then return to slaughter the sacrifice. Whoever does so, he acted according to our Sunna (tradition), and whoever slaughtered (the sacrifice) before the prayer, what he offered was just meat he presented to his family, and that will not be considered as Nusak (sacrifice)." (On hearing that) Abu Burda bin Niyar got up, for he had slaughtered the sacrifice before the prayer, and said, "I have got a six month old ram." The Prophet said, 'Slaughter it (as a sacrifice) but it will not be sufficient for any-one else (as a sacrifice after you). Al-Bara' added: The Prophet said, "Whoever slaughtered (the sacrifice) after the prayer, he slaughtered it at the right time and followed the tradition of the Muslims."