Musannaf Ibn Abi Shayba 3:533

Abu Bakr narrated to us, he said: Waki’ ibn al-Jarrah narrated to us, from Sufyan, from Salim Abu al-Nadr, from Bishr ibn Sa’id, from ‘Abdullah ibn Jahm, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you knew what is upon him of sin in passing in front of his brother while he is praying, he would have stood for forty."