The Laws of Prayer (The morning services) (44)

1. One may not pass within four cubits in front of one who is praying, as by doing this one is disturbing his concentration. Another reason is that the Divine Presence rests opposite the one who is praying, and if one passes in front of him it is as if one is interposing between him and the Divine Presence. (Shulchan Aruch and Mishne Berurah 102)

2. One is allowed to pass by the side of one who is praying. However, there are some who are stringent regarding the sides which are diagonally in front of him within four cubits as there also one disturbs his concentration. According to the Zohar one may never pass by the side of one who is praying. (ibid)