This story was narrated by Markandeya to Yudhishtira– it appears in the Vana Parva of Mahabharat.
Once there lived Kaushika, a young learned brahmin who led an ascetic life. One day he was sitting below a tree in penance. A female crane sitting on top of the tree let her droppings fall on Kaushika. Disturbed by this, Kaushika cast his angry eyes on the bird. Next minute the poor creature lay charred on the ground. Later in the day, Kaushika left for the nearby village to collect his bhiksha (alms). He entered a house and was told by the lady of the house to wait as she was engaged in some work. Kauskika waited for long and was angered by the treatment he was given. When the lady ultimately appeared, Kaushika was in a very agitated state and gave her a burning glance. He asked her why she did not cater to a Brahmin on priority. The lady responded that she was tending to here husband and that was her most important duty. Everything else would come later. In the end, she also added that she was no crane to be burned down by his anger. The young Brahmin was astonished how she came to know about an event which happened far away in the forest. She suggested that Kaushika should go to Mithila and meet a butcher who could answer his questions. Though a little reluctant to seek knowledge from a butcher, Kaushika was overcome with curiosity and decided to do as the lady suggested. He reached Mithila and found the Vyadha (butcher), who recognised him and was well aware of his purpose. Later, after finishing his work, Vyadha took Kaushika to his home and gave him a sermon on virtue, karma and a whole lot of other aspects of life. He talked about how one should focus on one’s work, irrespective of its nature. The enlightening speech of the Vyadha (butcher) is called ‘Vyadha-Gita’ and is considered a brilliant philosophical sermon. Vyadha also told Kaushika about his (Vyadha’s) previous life in which he accidentally killed a sage during a hunt and was cursed to be reborn as a butcher.
If you are interested in knowing more about what Vyadha spoke about, check out chapters 206 to 211 in Vana Parva, Mahabharata