Is reason a source of creed along with the Quran and Sunnah, and why did Ibn Taymiyyah use reason to refute the Jahmiyyah in his book "Bayan Talbis al-Jahmiyyah"?

The source of creed is the Qur'an and the Sunnah, not the intellect. The intellect is not used to infer unseen matters that have not been mentioned in them. A sound intellect testifies to what the Qur'an and Sunnah have brought. Therefore, scholars cite it and affirm its indication of what the revelation has spoken, and they use it to prove the invalidity of statements that contradict the Sharia. Sheikh Ibn Uthaymeen said: "Creed must be based on the Book of Allah and the Sunnah of His Messenger. There is no room for the intellect in it, except that the texts of Allah's perfection are attested to by intellects, even if they do not grasp the details of that."