The meat of an animal for which no blessing (Bismillah) was recited.
Aisha (may Allah be pleased with her) narrated:
The companions came to the Messenger of Allah (peace and blessings upon him) saying, “People who have just emerged from ignorance bring slaughtered meats to us, and we do not know whether they said Bismillah or did not.” They asked, "Should we eat [it]?" The Messenger of Allah (peace and blessings upon him) said, “Say Bismillah and eat.” [554]
COMMENTARY:
According to the Hanafi school of thought: Reciting Bismillah while slaughtering an animal is obligatory. The meat of an animal slaughtered by someone who deliberately omits Bismillah is not permissible to be eaten. If one slaughters it forgetting Bismillah, then it is permissible to eat.
According to the Shafi’i, Maliki, and Hanbali schools of thought: Reciting Bismillah while slaughtering an animal is not obligatory; it is recommended. The meat of an animal slaughtered by someone who does not recite Bismillah, whether deliberately or forgetfully, is permissible to be eaten. (Mealim al-Sunan, vol. 4, p. 122)