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
Ikrimah (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): "So eat of that over which the name of Allah has been mentioned" [Surah al-An'am: 118], "And do not eat of that over which the name of Allah has not been mentioned" [Surah al-An'am: 121]. But He made an exception to it: "And the food of those who were given the Scripture is lawful for you, and your food is lawful for them" [Surah al-Ma'idah: 5].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19153