Are the verses that suddenly come to mind, as if to warn of Allah's punishment for sins, from Allah or whisperings from Satan?

The occurrence of these verses is mostly not from the whispers of Satan. Rather, it may be from the inspiration of the angel assigned to the servant, to instill fear of Allah Almighty in him. The angel is a guardian of the servant in his wakefulness, sleep, and life; he comforts him, fights his enemy on his behalf, assists him, and casts good into his heart, urging him to believe in the truth. This is unlike Satan, who promises evil and disbelieves in the truth. And if the angel draws near to the servant, he speaks through his tongue with sound words, whereas if Satan draws near, he speaks with falsehood and obscenity, as Ibn al-Qayyim stated.