Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 29

Sanskrit:

ततश्च लब्धसंस्कारौ द्विजत्वं प्राप्य सुव्रतौ ।गर्गाद् यदुकुलाचार्याद्गायत्रं व्रतमास्थितौ ॥ २९ ॥

ITRANS:

tataś ca labdha-saṁskāraudvijatvaṁ prāpya su-vrataugargād yadu-kulācāryādgāyatraṁ vratam āsthitau

Translation:

After attaining twice-born status through initiation, the Lords, sincere in Their vows, took the further vow of celibacy from Garga Muni, the spiritual master of the Yadus.

Purport:

Both Śrīdhara Svāmī and Viśvanātha Cakravartī Ṭhākura explain the term gāyatraṁ vratam as the vow of brahmacarya, or celibacy in student life. Kṛṣṇa and Balarāma were playing the part of perfect students on the path of self-realization. Of course, in the modern, degraded age, student life has become a wild, animalistic affair filled with illicit sex and drugs.