What is required regarding the intention (Niyyah) during worship?

Primary text

It is obligatory to maintain the legal effect (hukm) of the intention, not its absolute reality (haqiqah). This means that one must not intend to cut off the intention. If one becomes oblivious to the intention or it slips away during the prayer, this does not invalidate the prayer because avoiding such distraction is impossible. Furthermore, the true reality of the intention is not considered during the act of worship, as evidenced by fasting and other acts. Evidence for this is the hadith where the Prophet (peace be upon him) stated that when the call to prayer is made, Satan flees, but when the second call is completed, he returns, whispering to the worshipper to remember things, such that one might forget how many units of prayer he has performed. This hadith is found in the agreed-upon collections (Muttafaq 'alayh) and reported by Malik in his Muwatta.