I heard the Apostle of Allah (peace_be_upon_him) say: There were two men among Banu Isra’il, who were striving for the same goal. One of them would commit sin and the other would strive to do his best in the world. The man who exerted himself in worship continued to see the other in sin. He would say: Refrain from it. One day he found him in sin and said to him: Refrain from it. He said: Leave me alone with my Lord. Have you been sent as a watchman over me? He said: I swear by Allah, Allah will not forgive you, nor will he admit you to Paradise. Then their souls were taken back (by Allah), and they met together with the Lord of the worlds. He (Allah) said to this man who had striven hard in worship; Had you knowledge about Me or had you power over that which I had in My hand? He said to the man who sinned: Go and enter Paradise by My mercy. He said about the other: Take him to Hell. AbuHurayrah said: By Him in Whose hand my soul is, he spoke a word by which this world and the next world of his were destroyed.
[Abu Dawud Book 41, Hadith 4883]