An elder Chernobyler chossid passing through Leningrad told the following story:
A chossid who was accustomed to fast from one Shabbos to the next was told by his teacher that it was unnecessary.
"But isn't it known," asked the chossid, "that when the Baal Shem Tov embarked on his solitary meditations, he fasted from Shabbos to Shabbos?"
"When the Baal Shem Tov left his home on motzaei Shabbos," his teacher replied, "to walk the mountains from Kitov to Kosov, he took along a sack filled with food. And when it came time to re-turn on Friday he realized that the sack was still full. His 'I have forgotten to eat my bread' resulted from deveikus to G-dliness."