Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 10
Sanskrit:
कौस्तुभव्यपदेशेन स्वात्मज्योतिर्बिभर्त्यज: ।तत्प्रभा व्यापिनी साक्षात् श्रीवत्समुरसा विभु: ॥ १० ॥
ITRANS:
kaustubha-vyapadeśenasvātma-jyotir bibharty ajaḥtat-prabhā vyāpinī sākṣātśrīvatsam urasā vibhuḥ
Translation:
Upon His chest the almighty, unborn Personality of Godhead bears the Kaustubha gem, which represents the pure spirit soul, along with the Śrīvatsa mark, which is the direct manifestation of this gem’s expansive effulgence.