Allah, glorified be His praise, says: "And the food of those who were given the Scripture is lawful for you." [Surah al-Ma'idah: 5]
Imam al-Shafi'i (may Allah have mercy on him) said: "And according to some of the people of tafsir from whom we have preserved (knowledge), their food means their slaughtered animals, and the reports also indicate that their slaughtered animals are lawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / Q19152