Are the thoughts and fantasies that cross my mind during supplication (Du'a) and prayer (Salat), which include thinking of other than Allah or His Messenger, and the smile that follows them, a form of demonic whispering (Waswasah) or are they from me? Am I held accountable for them, or have I, by that, committed an act of disbelief (Kufr)?

It is not permissible to delay prayer beyond its prescribed time except for a legitimate Shar'i excuse. Fantasies and whisperings are from Satan, and a Muslim must seek refuge in Allah and desist from them. These obsessive whisperings are not held against the servant as long as he does not act upon them or speak of them. They are evidence of the strength of faith and the soundness of belief, because considering them grave and recoiling from them is a sign of complete faith.