Abul-Juhaim `Abdullah bin Al-Harith (May Allah be pleased with him) said: The Messenger of Allah (PBUH) 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 (PBUH) said forty days, months or years.]
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.