Musnad Ahmad 1139:1

Waki' and 'Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Salim Abu al-Nadr, from Busr ibn Sa'id, that Zayd ibn Khalid sent to Abu Juhaym—'Abd al-Rahman said: Zayd ibn Khalid sent me to Abu Juhaym al-Ansari—[asking]: "What did you hear the Messenger of Allah, peace and blessings of Allah be upon him, say regarding a man who passes in front of a man while he is praying?" He said: I heard him say: "If one of you knew what [sin] is upon him for passing in front of a man while he is praying, it would be better for him to stand for forty—I do not know: a year, or a day, or a month—than to do that."