Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

Zaid bin Khalid sent word to Abu Juhaim Al-Ansari asking him: "What did you hear from the Prophet (saw) about a man when he is performing prayer?" He said: "I heard the Prophet (saw) saying: 'If anyone of you knew (how great is the sin involved) when he passed in front of his brother who is performing prayer, then waiting for forty'," (one of the narrators) said: "I do not know if he meant forty years, forty months, or forty days, 'would be better for him than that." Daif