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."
Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "There are three things whose earnest is earnest and whose jest is earnest: 1 marriage, 2 divorce, 3 taking back (raj'ah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14993