Sidq (truthfulness) and niyya (intention) are two names. Their essential characteristic is a truthful will. (Adāb, 140)
Regarding the nature of niyya, it is said: The servant's will is directed toward acting with one of the meanings (purposes). Because of this meaning, when the servant wishes to perform this act, this will becomes niyya either for Allah or for something other than Him. (Riʿāya, 144)
Niyya is to resolve to perform the act. The (Sufi) community has said: Niyya is to know the name and (the quality, the nature) of the act to be performed. (Lumaʿ, 303)
Types of khawāṭir (thoughts): Of the things that come to the qalb (the heart), that which is directed toward determining and considering good is called niyya. For these are either the khāṭir of the nafs (the lower self), or, due to envy, of the enemy...