Imam Malik (may Allah have mercy on him) said: Their repentance is not to be accepted. The Shaykh (may Allah have mercy on him) said: The view of those who say that repentance will be accepted is correct; as for what is hidden, Allah Almighty knows it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16856