Ibn Mas'ud (May Allah be pleased with him) reported:
One night I joined the Prophet (ﷺ) in his (optional) Salat. He (ﷺ) 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].
App reference: Book 9, Hadith 184
Arabic/English book reference: Book 9, Hadith 1174