Movement while praying and preventing someone from passing.

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.