"It will never reach Allah, neither their meat nor their blood," meaning: neither their meat nor their blood ascends to Allah. The polytheists used to slaughter for their idols, then splash their blood around the House (Ka'bah). "But what reaches Him is piety from you," meaning: it ascends to Him from those who have believed. "Thus have We subjected them to you that you may glorify Allah for that to which He has guided you." The Sunnah, when slaughtering or sacrificing, is to say: Bismillahi wallahu akbar (In the name of Allah, and Allah is the Greatest).

Surah Al-Hajj, verses 38-41.