Ali (may Allah be pleased with him) said: The apostate is to be given the chance to repent three times. Then he recited this verse: "Indeed, those who believed, then disbelieved, then believed, then disbelieved, then increased in disbelief" [Surah al-Nisa: 137].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16889