Ibn Mas'ud (May Allah be pleased with him) said:
One night I joined the Prophet (ﷺ) in his (optional) Salat. He prolonged the standing so much that I thought of doing something evil. He was asked: "What did you intend to do?" He replied: "To sit down and leave him".[Al-Bukhari and Muslim].
Arabic/English book reference: Book 1, Hadith 103