Is the hadith: "Whoever recites 'Qul Huwa Allahu Ahad' a hundred times in two rak'ahs on one of the nights of Rajab, it is as if he has fasted a hundred years in the way of Allah, and Allah will grant him a hundred palaces in Paradise, each palace next to a prophet from among the prophets" authentic?

This hadith is not found in the books of the Prophet's Sunnah. It is a fabrication attributed to the Messenger of Allah, peace and blessings be upon him, and bears the hallmarks of a forged narration. This is because it includes a tremendous reward for a small deed, or a severe warning for a minor sin, as Al-Suyuti mentioned in his Alfiyyah.