who passes in front of a praying person

Abul-Juhaim `Abdullah bin Al-Harith 
(May Allah be pleased with him) said: 
The Messenger of Allah (Sallallahu Alaihi Wasallam) said, 
"If the person who passes in front of a praying person, 
realizes the enormity of the sinfulness of this act, 
it will have been better for him to wait 
forty than to pass in front of him.''
[Al-Bukhari and Muslim].

[The narrator was not sure whether 
the Prophet (Sallallahu Alaihi Wasallam) said
forty days, months or years.]

Commentary:  
We learn from this Hadith that it is a great sin to pass before 
a person who is offering Salat. People should also take care that 
they do not offer Salat without placing a Sutrah in front of them. 
A Sutrah refers to anything that a person sets up in front of him; 
this could be a stick, another person praying in front of him or 
even a line which he can draw on the ground. The distance between 
the person offering his Salat and the Sutrah should not be more 
than approximately one meter and a half. If the distance is longer 
than this, then it is not unlawful to pass before someone offering Salat.