Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 41
Sanskrit:
कदाचिद् यमुनातीरे वत्सांश्चारयतो: स्वकै: ।वयस्यै: कृष्णबलयोर्जिघांसुर्दैत्य आगमत् ॥ ४१ ॥
ITRANS:
kadācid yamunā-tīrevatsāṁś cārayatoḥ svakaiḥvayasyaiḥ kṛṣṇa-balayorjighāṁsur daitya āgamat
Translation:
One day while Rāma and Kṛṣṇa, along with Their playmates, were tending the calves on the bank of the river Yamunā, another demon arrived there, desiring to kill Them.