Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 5

Sanskrit:

परमासन आसीनं कृष्णा कृष्णमनिन्दिता ।नवोढा व्रीडिता किञ्चिच्छनैरेत्याभ्यवन्दत ॥ ५ ॥

ITRANS:

paramāsana āsīnaṁkṛṣṇā kṛṣṇam aninditānavoḍhā vrīḍitā kiñcicchanair etyābhyavandata

Translation:

Faultless Draupadī, the Pāṇḍavas’ newly married wife, slowly and somewhat timidly approached Lord Kṛṣṇa, who sat on an exalted seat, and offered Him her obeisances.

Purport:

Śrīmatī Draupadī was so devoted to Kṛṣṇa that she herself was called Kṛṣṇā, which is the feminine form of the name, and Arjuna was also called Kṛṣṇa because of his devotion to the Lord. Similarly, the devotees of the modern Kṛṣṇa consciousness movement are often called “the Kṛṣṇas.” So it appears that the custom of addressing Kṛṣṇa’s devotees by His name has a long history.