What is the ruling on saying: "Indeed, Allah and His Messenger are truthful in their sayings, but Allah is more truthful than His Messenger Muhammad, may Allah's prayers and peace be upon him"?

Allah Almighty is far too great and majestic to be compared to His creation, or for His attributes to be likened to theirs. There is nothing whatsoever like unto Him. The Prophet, peace and blessings be upon him, rebuked a Bedouin who attempted to draw a comparison between Allah and His creation, clarifying Allah's immense greatness and that His Throne encompasses the heavens. Whoever utters words that imply a comparison between Allah and His Messenger must repent, and must differentiate between the glorification, transcendence, and sole devotion of worship due to Allah alone, and the belief, obedience, and reverence due to the Messenger.