Siman 552

(109.) Regarding the melancholy thoughts that confuse a person — and particularly during prayer — introducing doubts and confusions in one's thought about how one did something improper, and stumbled into some prohibition, G-d forbid: Rabbainu o.b.m. told me an incident of the Baal Shem Tov of blessed memory — that once the Baal Shem Tov stood to pray and was unable to pray, for a confusion fell in his thought, in that... ...in that he would smoke the lulkeh [pipe] beside a tallow candle — and it turned out he had stumbled into a tallow prohibition. And because of this he was unable to pray. And all that he wanted — to pray and to push away this thought and remove it from his mind — he was unable to do in any way, for it confused him every time, in that he had stumbled into such a prohibition — a tallow prohibition — until it was impossible for him to begin to pray. The Baal Shem Tov stood up and leapt and swore that he would forever smoke the lulkeh beside a tallow candle. And so it was — as the world tells: the Baal Shem Tov would smoke the lulkeh beside a tallow candle.