Virtues of the Prayer Hall (Sutra of the Musalla)

Bukhari :: Book 1 :: Volume 9 :: Hadith 489

Narrated Busr bin Said:

that Zaid bin Khalid sent him to Abi Juhaim to ask him what he had heard from Allah's Apostle about a person passing in front of another person who was praying. Abu Juhaim replied, "Allah's Apostle said, 'If the person who passes in front of another person in prayer knew the magnitude of his sin he would prefer to wait for 40 (days, months or years) rather than to pass in front of him." Abu An-Nadr said, "I do not remember exactly whether he said 40 days, months or years."