Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

When I came to Madinah, I prayed, "O Allah! let me have a righteous companion." So, I sat down with Abu Hurayrah (RA) and said to him "I requested Allah to let me have a righteous companion. So, narrate to me a Hadith that you may have heard from Allah's Messenger that Allah may benefit me with it." So, he said, "I heard Allah's Messenger (SAW) say, "The first thing a slave will be called to account from his deeds on the Day of Resurrection will be his salah. So, if it is correct then he wil succeed and earn deliverance, but if it is corrupted then he will fail and lose. If there is some shortcoming in his fard, the Lord Blessed and Elevated will say, 'Examine and see! HasMy slave any optional deeds that the shortcomings in the fard might be offset.' Then all his deeds will be (recompensed) in that way.'