Abu Dawud Book 2, Hadith Number 1512.
Chapter : On seeking pardon and repentance.
Narated By Zayd, the client of the Prophet : The Prophet (PBUH) said: If anyone says: “I ask pardon of Allah than Whom there is no deity, the Living, the eternal, and I turn to Him in repentance,” he will be pardoned, even if he has fled in time of battle.