It is forbidden to pass in front of a person who is praying (i.e., between him and his sutrah).
There are many hadith which forbid passing between a person and his sutrah, and describe such an act as a major sin.
Busr ibn Sa’id says that Zaid ibn Khalid sent him to Abu Juhaim to ask him what he had heard from the Prophet sallallahu alehi wasallam concerning passing in front of someone who is praying. He said that the Messenger of Allah said: “If one knew [the sin] of passing in front of one who is praying, he would rather wait forty […] than to pass in front of him.” This is related by the group.
Zaid ibn Khalid relates that the Messenger of Allah said: “If the one who passes in front of one who is praying knew what was upon him [of sin], it would be better for him to stand [and wait] for forty autumns than to pass in front of him.” This is related by al-Bazzar with a sahih chain.
Ibn al-Qayyim writes: “Ibn Hibban and others say that the prohibition mentioned in this hadith applies when one is praying with a sutrah. If one is praying without a sutrah, it is not forbidden to pass in front of him. As a proof, Abu Hatim [i.e. ibn Hibban] argues by the hadith, in his sahih, from al-Mutalib ibn Abi Wid’ah who said: ‘I saw the Prophet, when he finished the circumambulation [of the Ka’bah], he went to the end of the circuit and he prayed two rak’at and there was nothing between him and the people who were circumambulating.” Abu Hatim says: “This report proves that it is permissible to pass in front of a person who is praying but without a sutrah. In this lies a clear proof that the warning concerning passing in front of one who is praying refers only to one who is praying toward his sutrah and does not refer to one who does not have a sutrah.” Abu Hatim explains that the Prophet’s prayer was without anything between him and the people circumambulating the ka’bah. At the end of the hadith of al-Mutalib, he records: “I saw the Prophet of Allah offering salah facing the black stone and the men and women were passing in front of him and there was no sutrah between him and them. In ar-Raudah anNadiyah, it is stated that if one has no sutrah or is far away from the sutrah, then he is not to keep anyone from passing in front of him and it is not forbidden to pass in front of him although it is preferred not to do so.