Kansa informed by Nārada of the existence of Krishna and Balarāma; he sends Kesin to destroy them and Akrura to bring them to Mathura - Verse 43
Vishnu Purana (GRETIL; M. M. Pathak critical edition) · 4 · Verse 15.43
Sanskrit Original
तेषां च प्रद्युम्नचारुदेष्णसाम्बादयस त्रयोदश प्रधानाः। प्रद्युम्नो ऽपि रुक्मिणस तनयां ककुद्वतीं नामोपयेमे। तस्याम अस्यानिरुद्धो जज्ञे। अनिरुद्धो ऽपि रुक्मिण एव पौत्रीं सुभद्रां नामोपयेमे। तस्याम अस्य वज्रो ऽभवत। वज्रस्य प्रतिबाहुस तस्यापि सुचारुः। एवम अनेकशतसाहस्रपुरुषसंख्यस्य यदुकुलस्य पुरुषसंख्या वर्षशतैर अपि ज्ञातुं न शक्यते।।
teṣāṃ ca pradyumnacārudeṣṇasāmbādayas trayodaśa pradhānāḥ| pradyumno 'pi rukmiṇas tanayāṃ kakudvatīṃ nāmopayeme| tasyām asyāniruddho jajñe| aniruddho 'pi rukmiṇa eva pautrīṃ subhadrāṃ nāmopayeme| tasyām asya vajro 'bhavat| vajrasya pratibāhus tasyāpi sucāruḥ| evam anekaśatasāhasrapuruṣasaṃkhyasya yadukulasya puruṣasaṃkhyā varṣaśatair api jñātuṃ na śakyate||
Among them are the thirteen principal ones, including Pradyumna, son of Rukmini, who married Kakudvati. Aniruddha was born from her. Aniruddha also married Subhadra, daughter of Rukmini. From her, Vajra was born, and Vajra had a beautiful son. Thus, it is impossible to know the number of people in the Yadu dynasty, even after hundreds of years.