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
عن أبي جُهَيْمِ بْنِ الْحَارِثِ بْنِ الصِّمَّةِ الأنصاري رضي الله عنه مرفوعاً: «لو يعلم المار بين يدي الْمُصَلِّي ماذا عليه لكان أن يَقِفَ أربعين خيرا له من أن يَمُرَّ بين يديه». قال أَبُو النَّضْرِ: لا أدري: قال أربعين يوما أو شهرا أو سنة.
Explanation
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.
Source
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