Musannaf Ibn Abi Shayba 33:266

Abu al-Ahwas narrated to us, from Simak, from al-Nu'man ibn Bashir, who said: Umar was asked about the "sincere repentance" (al-tawbah al-nasuh). He said: The sincere repentance is that the servant repents from the evil deed, then he never returns to it ever.