What is the ruling on doubting the truthfulness of the Messenger of Allah, peace and blessings be upon him?

Doubt concerning the truthfulness of the Prophet Muhammad, peace and blessings be upon him, invalidates faith, because it is doubt concerning one of the two pillars of the Shahada. Since the doubt occurred in childhood and then the questioner repented and became certain of the Messenger's truthfulness, there is nothing against him, because repentance erases what came before it. It appears that the questioner is suffering from obsessive whispers in this regard, and he must turn away from them and seek Allah's help.