Hadith: "If the person passing in front of someone praying knew the extent of the guilt he is incurring, he would prefer to stand for forty rather than pass in front him."* Abu an-Nadr said: I am not sure if he said forty days, months, or years.

عَنْ بُسْرِ بْنِ سَعِيدٍ، أَنَّ زَيْدَ بْنَ خَالِدٍ الْجُهَنِيَّ رَضيَ اللهُ عنه، أَرْسَلَهُ إِلَى أَبِي جُهَيْمٍ رَضيَ اللهُ عنه، يَسْأَلُهُ مَاذَا سَمِعَ مِنْ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فِي الْمَارِّ بَيْنَ يَدَيِ الْمُصَلِّي؟ قَالَ أَبُو جُهَيْمٍ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«لَوْ يَعْلَمُ الْمَارُّ بَيْنَ يَدَيِ الْمُصَلِّي مَاذَا عَلَيْهِ لَكَانَ أَنْ يَقِفَ أَرْبَعِينَ خَيْرًا لَهُ مِنْ أَنْ يَمُرَّ بَيْنَ يَدَيْهِ» قَالَ أَبُو النَّضْرِ: لَا أَدْرِي قَالَ: أَرْبَعِينَ يَوْمًا أَوْ شَهْرًا أَوْ سَنَةً؟
[صحيح] - [متفق عليه] - [صحيح مسلم: 507]

Busr ibn Sa‘īd reported that Zayd ibn Khālid al-Juhani (may Allah be pleased with him) sent him to Abu Juhaym (may Allah be pleased with him) to ask him about what he heard from the Messenger of Allah (may Allah's peace and blessings be upon him) about the one who passes in front of the praying person. Abu Juhaym said: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"If the person passing in front of someone praying knew the extent of the guilt he is incurring, he would prefer to stand for forty rather than pass in front him." Abu an-Nadr said: I am not sure if he said forty days, months, or years.
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Muslim - 507]

Explanation

The Prophet (may Allah's peace and blessings be upon him) warned against passing in front of a person performing an obligatory or supererogatory prayer, and that if he knew the extent of guilt he is incurring by doing this deliberately, he would choose to stand for forty rather than to pass in front of him. Abu an-Nadr, the Hadīth narrator, said: I am not sure if he said forty days, months, or years.

Benefits from the Hadith

1. The prohibition of passing in front of someone who is praying, whether he has no Sutrah (barrier), or passing between him and his barrier if he does have one.
2. Ibn Hajar said: There is a difference of opinion regarding the specific limit of this prohibition. It was said: if someone passes between the praying person and the area of his prostration. And it was also said: if the passer goes between him and a distance of three cubits. And another view holds: between him and the range of a stone's throw.
3. As-Suyūti said: What is meant by "passing in front of" is to pass horizontally across in front of the one praying. But if a person walks in front of him heading toward the Qiblah direction (direction of the Ka‘bah), then he is not included in the warning.
4. It is preferable for the one who is praying not to pray in places where people commonly pass through or in areas where people must walk, so as not to expose his prayer to deficiency or expose the passersby to sin. He should also place a barrier between himself and those passing in front of him.
5. It is understood from this that the sin resulting from disobedience in the Hereafter—no matter how small—is greater than any hardship in this world, no matter how severe.