The following Hadith has been reported in Bukhaari Shareef and there are similar narrations all the other SIX major books of Ahaadith, which highlights the gravity of the sin of those who walk in front of others performing Salaat:
“It has been narrated on the authority of Busr bin Saeed that Zaid bin Khalid sent him to Abi Juhaim to ask him what he had heard from Rasulullah (sallallahu alaihi wasallam) about a person passing in front of another person who was praying. Abu Juhaim replied, “Rasulullah (sallallahu alaihi wasallam) said, ‘If the person who passes in front of another person in prayer knew the magnitude of his sin he would prefer to wait for 40 (days, months or years) rather than to pass in front of him.” Abu An-Nadr said, “I do not remember exactly whether he said 40 days, months or years.”
“Nabi (sallallahu alaihi wasallam) said: “If anyone of you knew (how great is the sin involved) in passing in front of his brother while he is performing prayer, waiting for one hundred years would be better for him than one step that he takes.’” [Sunan Ibn Majah]
During the initial period, Nabi (sallallahu alaihi wasallam) has been reported to have said that if anyone passes in front of another who is performing Salaat, then he (the person performing Salaat) must fight with the passer-by, because he is the devil. In other words, only a shaitaan would be so disrespectful as to pass in front of another who is performing Salaat.
In essence, it is HARAAM to walk in front of a person who is performing Salaat. One should either wait until the other is completed his Salaat or walk around him, so as not to pass in front whilst he is engaged in Salaat.
Brothers should be careful to NEVER EVER walk in front of another who is performing Salaat. If a person realised the gravity of the sin involved in walking in front of another who is performing Salaat, and if he knew the punishment in store for this sin in the Aakhirah, he would rather wait for a HUNDRED YEARS for his fellow brother to complete his Salaat, before walking in front of him.