Allah, exalted be His praise, says: "Whoever disbelieves in Allah after his belief, except one who is compelled while his heart is at rest with faith, but whoever opens his breast to disbelief" (the verse) [Surah al-Nahl: 106].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / Q16896