Musannaf Ibn Abi Shayba 3:531

Abu Abd al-Rahman Baqi ibn Makhlad narrated to us, he said: Abu Bakr Abdullah ibn Muhammad ibn Abi Shayba narrated to us, he said: Muhammad ibn Fudayl narrated to us, from Muhammad ibn Ishaq, from Abd al-Rahman ibn al-Aswad, from his father, who said: When someone would pass in front of Ibn Mas’ud while he was praying, he would embrace him until he turned him back, and he would say: "Indeed, a person passing in front of one [praying] cuts off half of the person's prayer."