Imam al-Shafi'i (may Allah have mercy on him) said: "Allah Almighty has mentioned divorce in His Book by three names: talaq (divorce), firaq (separation) and sarah (release). So whoever addresses his wife and uses any one of these names specifically for her, divorce becomes binding upon him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / Q14993