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
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:

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.