In his story The Sophisticate and the Simpleton Rabbi Nachman mentions the Devil
(Source from Wikisource)
Meanwhile someone came and said, "The Devil [Yid. Toivl, Heb. Azazel] has sent for you [plural]." The simple man trembled severely and ran and told his wife with great fear that the Devil had sent for him. She gave him an advice, that he should send for the baal shem. He sent for him; the baal shem came and gave him kame`as [amulets containing holy names] and [other] protections and told him that now he need no longer fear at all. He had great faith in this.
Rabbi Aryeh Kaplan in his book The Lost Princess & Other Tales of Rabbi Nachman of Breslov comments:
Devil 'Teivel' in Yiddish; 'azazel' in Hebrew. The term 'azazel' is found in the Torah, relating to the place where the Yom Kippur scapegoat is sent (Leviticus 16:10). In general, it denotes the forces of evil (Rimzei Maasioth)
Where is the earliest mention of the Devil (or azazel connoting the Devil) in Jewish Literature?