Abu Dawud Book 2, Hadith Number 1394.
Narated By Abdullah ibn Amr : A man came to the Apostle of Allah (PBUH) and said: Teach me to read the Qur’an, Apostle of Allah.
He said: Read three surahs which begin with A.L.R. He said: My age is advanced, my mind has become dull (i.e. memory has grown weak), and my tongue has grown heavy). So he said: Then read three surahs which begin with H.M. He repeated the same words. So he said: Read three surahs which begin with the “Glorification of Allah”. But he repeated the same excuse. The man then said: Teach me a comprehensive surah, Apostle of Allah. The Prophet (PBUH) taught him Surah (99). “When the Earth is shaken with her earthquake”. When he finished it, the man said: By Him Who sent you with truth, I shall never add anything to it. Then man then went away.
The Prophet (PBUH) said twice: The man received salvation.