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 wait for] 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]