(And eat not of that whereon Allah's name hath not been mentioned…) [6:121].
The idolaters said: “O Muhammad, tell us: 'When a sheep dies, who has killed it?' He said: 'Allah killed it!' They said: 'How is it then that that which is killed by you and your Companions is lawful, that which is killed by a dog or bird of prey is lawful but that which is killed by Allah is unlawful?' And so Allah, exalted is He, revealed this verse”.
Said 'Ikrimah: “When Allah, exalted is He, revealed the unlawfulness of the meat of carrion, the Magians among the people of Persia wrote to the idolaters of Quraysh, who were their allies in the pre-Islamic period and kept contact with each other, that Muhammad and his Companions claim that they follow the command of Allah but then say that what they slaughter is lawful and that which is slaughtered by Allah is unlawful. This caused doubt in the minds of some Muslims, and so Allah, exalted is He, revealed this verse”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou