Once, Matali — the charioteer of Indra — was searching for a bridegroom for his daughter Gunakesi, who was celebrated all over the world for her beauty. Even after searching among gods, daityas, gandharvas, men and rishis, he could find no suitable husband. He then made a journey to the world of the Nagas.
On his way, he encountered the great Rishi Narada. Matali informed Narada of his mission, and the well-travelled Narada — who knew all the residents of the nether world — agreed to join him in his quest. After journeying through various regions of the nether world, Matali found a Naga youth by the name of Sumukha, whom he found suitable for his daughter.
On enquiring about Sumukha's lineage, Narada revealed that Sumukha was born in the race of Airavata and that his father Chikura had been killed by Garuda. When Narada told Sumukha's grandfather Aryaka of Matali's desire, he became both happy and sad. He said it was an honour to have such an alliance — but they lived in fear, for when Garuda killed Sumukha's father, he had also promised to kill Sumukha within a month.
Matali, being a close friend of Indra, suggested they all go to Indra and seek his help. When they arrived, they found Lord Vishnu also present. On hearing their story, Lord Vishnu asked Indra to give Amrutham to Sumukha and make him immortal like the other gods. Though Indra had once found that even his Vajra could do no damage to Garuda, he was reassured by Lord Vishnu and gave Amrutham to the Naga. Having drunk Amrutham and lost all fear of Garuda, Sumukha married Matali's daughter and returned home.
When the mighty Garuda heard what had happened, he was inflamed with rage. He came to Indra and demanded: "How dare you give away my food — the Naga Sumukha?" Garuda said: "Although I am of immeasurable strength and more powerful than you, I still show you respect as king of the devas. Through my strength I can carry the weight of three worlds without fatigue — yet out of respect for you, I carry your younger brother Vishnu on my back and protect his flag. There is no one in the whole universe capable of bearing such a heavy burden."
Lord Vishnu, who was also listening, smiled and said to Garuda: "You are very weak, and yet you speak such proud words. All the three worlds united cannot bear my weight — it is I alone who bear my own weight. If you are truly capable of bearing the weight of only my right arm, then perhaps your words hold truth."
Saying this, Lord Vishnu gently placed his right arm on Garuda's shoulders. At the mere touch of that arm, Garuda fell down, became breathless and lost his senses. He felt as though one arm alone weighed as much as the complete earth along with all its mountains. Gasping for life, Garuda began losing all his feathers. Immediately he bowed down to the great Lord and asked for forgiveness. "This strength of yours was never known to me," he said. "Please forgive this winged creation."
Lord Vishnu was pleased. He told Garuda never to boast of his strength again — and also asked him to carry the Naga Sumukha on his breast.
This story was told by the Kanwa Rishi to Duryodhana and others in the Kuru assembly — to let them know that Krishna is none other than Lord Vishnu himself, and that the Kauravas stand no chance against the Pandavas who are ever protected by him.