Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 1
मैत्रेय उवाचविजिताश्वोऽधिराजासीत्पृथुपुत्र: पृथुश्रवा: ।यवीयोभ्योऽददात्काष्ठा भ्रातृभ्यो भ्रातृवत्सल: ॥ १ ॥
maitreya uvācavijitāśvo ’dhirājāsītpṛthu-putraḥ pṛthu-śravāḥyavīyobhyo ’dadāt kāṣṭhābhrātṛbhyo bhrātṛ-vatsalaḥ
The great sage Maitreya continued: Vijitāśva, the eldest son of Mahārāja Pṛthu, who had a reputation like his father’s, became emperor and gave his younger brothers different directions of the world to govern, for he was very affectionate toward his brothers.
After describing the life and character of Mahārāja Pṛthu in the previous chapter, the great sage Maitreya began to speak about the sons and grandsons in the genealogical line of the Pṛthu dynasty. After the death of Mahārāja Pṛthu, his eldest son, Vijitāśva, became emperor of the world. King Vijitāśva was very affectionate toward his younger brothers, and therefore he wanted them to rule different directions of the world. From time immemorial the eldest son generally becomes king after the death of the previous king. When the Pāṇḍavas ruled the earth, Mahārāja Yudhiṣṭhira, the eldest son of King Pāṇḍu, became emperor, and his younger brothers assisted him. Similarly, King Vijitāśva’s younger brothers were appointed to govern the different directions of the world.