Musnad Ahmad 507:1

I read to Abd al-Rahman: Malik, from Abu al-Nadr, the freed slave of Umar ibn Ubayd Allah, from Busr ibn Sa'id, that Zayd ibn Khalid al-Juhani sent him to Abu Juhaym to ask him: What did he hear from the Messenger of Allah, may Allah bless him and grant him peace, regarding the one who passes in front of a person praying, what is upon him? Abu Juhaym said: The Messenger of Allah, may Allah bless him and grant him peace, said: "If the one who passes in front of a person praying knew what was upon him, it would be better for him to stand for forty than to pass in front of him." Abu al-Nadr said: "I do not know whether he said forty days, or forty months, or forty years."