Ba'al Shem Tov, Emor 7/בעל שם טוב על התורה·אמור ז׳

1At first, the Baal Shem Tov was lenient about chadash. Once, though, he was traveling with Reb Yechiel, who had just been appointed rabbi of Horodnah, and saw that the latter was stringent in the matter, and he too began to be stringent.Midrash Pinchas HaChadash 83