Abu Juhaym ibn al-Hārith ibn As-Simmah al-Ansāri (may Allah be pleased with him) reported directly from the Messenger of Allah (may Allah's peace and blessings be upon him) that he said: "If the one passing in front of a person who is praying would know the gravity of his sin, it would have been better for him to stand for forty than to pass in front him." Abu An-Nadr said: "I am not sure if he said forty days, months, or years."
The praying person is standing before his Lord, conversing with Him in privacy and calling upon Him. If someone passes by him in this state, he interrupts this private conversation and disturbs his worship. That is why it is a serious sin on the part of whoever causes disturbance of a person's prayer by passing in front of him. So the Messenger of Allah (may Allah's peace and blessings be upon him) informed us that if a person would know the consequences of his passing; meaning: the sin and misdeed that he incurs, he would have preferred to stand in his place for extended periods of time than to pass in front of someone during his praying. Therefore, a Muslim must guard against this action and avoid it.