Killing People In The Masaajid-[Shaikh Rabee Reminds Us That This Is One of The Deeds of The Khawaarij (Devils)]
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Rabee Bin Haadi [may Allaah preserve him] said: The Khawaarij kill the people, whilst they are in Salaah. They [carefully determine] the time of the Salaah and kill people in their Masaajid. This has taken place in some of the lands- they [i.e. khawaarij] launch an attack and kill them, whilst they [i.e. the people] are in Salaah.
[Source: الذَّرِيعة إلى بيان مقاصد كتاب الشريعة (See 1st paragraph) 1/135]