Tasmiyah - It's Importance In Slaughtering An Animal

Upon slaughtering an animal, it is fardhu (religously obligatory) upon a Muslim to recite the name of Allah.

If he did not recite the name of Allah intentionally, the meat of the animal will not be Halal. (Surah Al-An'aam: Verse 121)

If he forgot or was under pressure, he will be excused and the meat of such an animal will be Halal.