Busr bin Sa'id narrated: Sayyiduna Abu al-Juhaym (may Allah be pleased with him), the son of the sister of Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him), sent me to Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) to ask him what he had heard from the Prophet (peace and blessings be upon him) about one who passes in front of someone praying. So I asked him, and he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "For one of you to remain standing for forty would be better for him than to pass in front of someone praying." The narrator did not remember whether he (peace and blessings be upon him) said forty years, forty months or forty days.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5217