My zeide described an occasion when the chassidim in Liozna wished to observe the Alter Rebbe’s eating habits.
Late one evening, watching stealthily through a window, they saw someone bring him a bowl of buckwheat kasha. When the Rebbe, reaching in to take a handful, said with a nigun: “It is necessary to take a kometz (the handful of the meal-offering in the Beis Hamikdash).”
The onlookers retreated in terror.