Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 33

Sanskrit:

यस्मिन् कृष्णो दिवं यातस्तस्मिन्नेव तदाहनि ।प्रतिपन्नं कलियुगमिति प्राहु: पुराविद: ॥ ३३ ॥

ITRANS:

yasmin kṛṣṇo divaṁ yātastasminn eva tadāhanipratipannaṁ kali-yugamiti prāhuḥ purā-vidaḥ

Translation:

Those who scientifically understand the past declare that on the very day that Lord Śrī Kṛṣṇa departed for the spiritual world, the influence of the Age of Kali began.

Purport:

Although technically Kali-yuga was to begin during the time of Lord Kṛṣṇa’s presence on earth, this fallen age had to wait meekly for the departure of the Supreme Personality of Godhead.