Muwatta Malik #366

Yahya related to me from Malik from Abu'n Nadr, the mawla of Umar ibn Ubaydullah from Busr ibn Said that Zayd ibn Khalid al-Juhani sent him to Abu Juhaym to ask him what he had heard from the Messenger of Allah, may Allah bless him and grant him peace, about passing in front of someone praying. 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 man praying knew what he was bringing upon himself it would be better for him to stop for forty than to pass in front of him.' " Abu'n-Nadr said, "I do not know whether he said forty days or months or years."