Is there any mention in the Prophetic Sunnah of the appellation "the one who knows Allah" (al-'arif billah) or "the scholar of Allah" (al-'alim billah)? And is it an innovation?

It is permissible to use titles such as "the Gnostic of God" (al-'Arif billah) for those who deserve them, namely the practicing scholars who adhere to the Sunnah and fear God. These titles have been used by many people of knowledge and Sunnah, such as Ibn Taymiyyah, Ibn al-Qayyim, and al-Ghazali, to describe degrees of faith, certainty, and a servant's knowledge of his Lord, and to distinguish the knowing believer from the mere imitator. It is not necessarily the case that these titles are innovations of the Sufis or the misguided sects, as they exploit legitimate religious terms. The Gnostic of God's will is united with God's will, and he does not pay attention to the blame of blamers. It is advisable to abandon argumentation and contention in religious matters.