It is not permissible to pray in slippers when there is harm
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The third question of Fatwa No. ( 18254 )
Q 3: What is the ruling in the religion regarding one who performs the prayer wearing shoes, even though there is carpeting and a prayer mat present, and yet he performs the prayer while wearing the shoes?
A 3: Praying in clean slippers is a recommended practice (sunnah), unless this leads to a concern, such as soiling the mosque’s carpet, in which case one should not pray in them, in order to avoid harm. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Q 3: What is the ruling in the religion regarding one who performs the prayer wearing shoes, even though there is carpeting and a prayer mat present, and yet he performs the prayer while wearing the shoes?
A 3: Praying in clean slippers is a recommended practice (sunnah), unless this leads to a concern, such as soiling the mosque’s carpet, in which case one should not pray in them, in order to avoid harm. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.