Musnad Ahmad 456:23

Sufyan narrated to us, from Salim Abu al-Nadr, the freed slave of Umar ibn Ubayd Allah ibn Ma'mar, from Busr ibn Sa'id, who said: Abu Juhaym, the nephew of Ubayy ibn Ka'b, sent me to Zayd ibn Khalid to ask him what he heard regarding the one who passes in front of a person praying. He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "For him to stand for forty—I do not know whether it is days, or months, or years—is better for him than to pass in front of him."