Please explain the concept of shirayim (literally, leftovers) wherein the followers of a chassidic Rebbe will receive portions of his food to it, these leftovers are considered holy or special in some way. Where does the concept of this ritual come from? Why is it only performed on shabbat and holidays? Is it specific to food (if so, why)? What are some of the first documented cases of shirayim being given out?
Here you have some answers to your question [in Hebrew]: