[The narrator was not sure whether the Prophet (PBUH) 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.