Abu 'Abdu'r-Rahman 'Abdullah ibn 'Umar ibn al-Khattab said that the Prophet, may Allah bless him and grant him peace, said, "Allah the Mighty and Majestic accepts the repentance of His servant as long as long as his death-rattle has not begun."

Riyadh-as-Saliheen by Imam Al-Nawawi, volume 1, hadith number 18 - Sunni Hadith