Abu Dawud Book 36, Hadith Number 5061.

Narated By Muslim at-Tamimi : The Apostle of Allah (PBUH) told him secretly: When you finish the sunset prayer, say: ‘O Allah, protect me from Hell” seven times; for if you say that and die that night, protection from it would be recorded for you; and when you finish the dawn prayer, say it in a similar way, for if you die that day, protection from it would be recorded for you. AbuSa’id told me that al-Harith said: The Apostle of Allah (PBUH) said this to us secretly, so we confine it to our brethren.