"I was prevented from praying the Qiyaam al-Lail (the night prayers) for 5 months because of a sin I had committed." He was asked: "What was that sin?" He said: "I saw a man crying and I said to myself: This is hypocrisy (showing off)."

Sufyaan al-Thawree رحمه الله [Ihyaa `Ulum al-Deen (2/200)]