Crossing In Front Of A Person Who Is Engaged In Salaah

There are severe warning mentioned in the Ahadith of Rasulullah Sallallahu Alayhi Wa Sallam in regards to crossing a person engaged in Salaah.

Rasulullah Sallallahu Alayhi Wa Sallam is reported to have said, Had a person known the sin of crossing in front of a Musalli, then standing for forty would have been better for him than crossing him. Abun Nadar (the narrator of the Hadith) states, I don’t know whether he said forty days, or forty months or forty years.
(Mu’atta)

In other words, a person would even prefer waiting for forty years if he has to, but will not cross in front of a Musalli due to the gravity of the sin. In another narration of Bukhari Sharif;

Rasulullah Sallallahu Alayhi Wa Sallam is reported to have said, If anyone passes in front of any of you whilst performing Salaah prevent him, if he refuses (to comply) then prevent him (for the 2nd time), if he refuses (to comply) then fight him for verily he is Shaytaan.
(Bukhari)

In this narration, a person that passes before a Musalli that is engaged in Salaah is referred to as Shaytaan.

There are two categories of Masajid, one is referred to as Masjid-e-Kabeer (big Masjid which is +-334sqm or more) whilst the other is referred to as Masjid-e-Sagheer (a small Masjid measuring less than 334sqm). 

In a Masjid-e-Kabeer, one may cross from a distance of two Saffs ahead of the Musalli (as the gaze of a Musalli generally doesn’t not exceed this distance), whilst it is Makrooh-e-Tahreemi to cross a Musalli that is engaged in Salaah in a Masjid-e-Sagheer. One should wait for the Musalli to terminate his Salaah in a Masjid-e-Sagheer and then only cross him. 

However, if a particular Musalli generally takes long in performing his Sunan and Nawaafil then he should not stand at such a place where he becomes an obstruction to other Musallis that may want to leave earlier. He should perform his Salaah behind a pillar or place a Sutra (such as a chair) before him so that others may cross by without difficulty. 

There is no harm in performing Salaah ahead or in front of a Musalli that is busy engaged in Salaah. The
prohibition is for crossing in front of him.