بسم الله الرحمن الرحيم
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What is a sutrah?
The 'sutrah' [partition] is an object to be placed in front of the praying person so serve as a partition or barrier from people who may want to walk in front of him or her.
What can be used as a sutrah?
The Prophet (pbuh) said: “When any one of you prays, let him place a sutrah for his prayer, even if it is an arrow.” [Ahmad] The Prophet (pbuh) also said: “When any one of you prays, let him face towards something, and if he cannot find anything, let him use a stick, and if he does not have a stick, let him draw a line, then it will not matter whatever passes in front of him.” [Ahmad]
Is a sutrah obligatory?
It is mustahabb [recommended] as opposed to obligatory as it is reported by Abu Dawud that the Prophet (pbuh) prayed in the desert without one.
"It is mustahabb according to the Hanafis, and the well known view of the Maalikis, for the imam and the person praying alone if they think that people will walk in front of them. Otherwise the sutrah is not prescribed in those cases. And it was narrated that Maalik enjoined it in all cases. This was also the view of Ibn Habeeb and was favoured by al-Lakhmi.
The Shaafa’is said that it is Sunnah in all cases, and they did not mention any limits.
The Hanbalis said: The sutrah is Sunnah for the imam and the person who is praying alone, even if they do not fear that anyone will pass in front of them." [Islamqa.com]
Thus a person who is praying in congregation, if he is not the imaam, does not need to put a sutrah in front of him. Allah knows best.