Mufti Abdullah Patel of Mufti Says rules that a person may use one hand to turn off their mobile phone while performing prayers without invalidating the prayer:

Fatwa, posted 4.22.2010, from United Kingdom, in:
Religious Authority: 
Mufti Abdullah Patel
Website URL: 
http://www.muftisays.com
Fatwa Question or Essay Title: 
Mufti Abdullah Patel of <i>Mufti Says</i> rules that a person may use one hand to turn off their mobile phone while performing prayers without invalidating the prayer:
Websites and Institutions: 
Mufti Says, Darul Uloom, (London, England, U.K.)

If one uses one hand to turn off the phone, then the prayer is not invalidated. That said, one should turn the phone off prior to praying.

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