Movement while praying and preventing someone from 
passing.
Question

                What should one do if one is distracted during the prayer by something 
happening in ones environment, such as someone knocking on the 
door, or a child in danger, e.g. playing with an electrical outlet, etc., 
perhaps requiring him to move?
Praise be to Allah.If a person praying needs to do something relatively minor during 
  prayer, like opening a door and the like, it is OK as long as his direction 
  does not change from the qibla (direction of prayer). The evidence for 
  this is what Abu Dawood narrated that Aa'isha (may Allah be 
  pleased with her), said, "The Messenger of Allah  
  (peace be upon him), used to pray with the door closed, so I arrived and asked 
  him to open it, so he came and opened it and returned to his prayer" (the 
  narrator mentioned that the door was in the direction of the qibla).1
The same is true if a mother needed, while praying, to save her 
  child from danger or harm and the likesmall movement to the right or left, 
  or forward or backward, does not harm the prayer. Similarly if the ridaa 
  (upper body cover) falls off, the praying person can pick it up and if the izar 
  (lower body cover) became loose, then he can tighten it up. The sharia 
  (laws of Islam) have allowed for the praying person in some cases excessive 
  movement even if he were to change direction from qibla, as evident in 
  the hadith of Abu Huraira, (may Allah be pleased with him), where he 
  said: "The Messenger of Allah  (peace be upon 
  him), said: Kill the two black things while in prayer: the snake and scorpion. 
  "2
Also, preventing a person from passing in the place of prostration 
  by pushing (or even fighting him if he insists) is prescribed, as related in 
  Bukhari from hadith Abi Saleh Al-Samman, who said: "I saw Aba 
  Saeed Al-Khudri on Friday praying with something in front of him to protect 
  him from [passing] people, and a young man from Bani Abi Muaet wanted to pass 
  in front of him in the place of prostration. So Abu Saeed pushed the mans 
  chest, so the man looked [around] and did not find any place from which to walk 
  except in front of him, so he tried to pass in front of him again. Abu Saeed 
  pushed him harsher than the first time, so he cursed Abu Saeed. Then the young 
  man went to Marwan and complained about Abu Saeeds action and Abu Saeed 
  entered after him at Marwans place so Marwan said: What is the matter 
  with you, doing that to your brothers son oh Abu Saeed.? He said: 
  I heard the Prophet  (peace be upon him) saying: 
  "If one of you prays with something in front of him protecting him from 
  [passing] people, and someone wants to pass in front of him, then he should 
  push him, and if he refuses to [turn back], then fight him, for he is a shaitan 
  (satan)."3
From the book What Should You Do in 
  the Following Situations... ?
1 Sunan Abu Dawood, number 
  922, and Sahih Sunan Abu Dawood 815.2 Sunan Abu Dawood, 
  number 921, and Sahih Sunan Abu Dawood 814.3 Narrated al-Bukhari 
  Fath 1/582.