Hadith 1174

Ibn Mas'ud (May Allah be pleased with him) reported: One night I joined the Prophet (PBUH) in his (optional) Salat. He (PBUH) prolonged the Qiyam (standing) so much that I made up my mind to commit an act of wrong. He was questioned: "What did you intend to do?" He replied: "I intended to sit down and stop following him (in Salat)."

[Al-Bukhari and Muslim].