Being very pleased by the unalloyed devotion of Mahārāja Ambarīṣa, the Supreme Personality of Godhead gave the King His disc, which is fearful to enemies and which always protects the devotee from enemies and adversities.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तस्मा अदाद्धरिश्चक्रं प्रत्यनीकभयावहम् । एकान्तभक्तिभावेन प्रीतो भृत्याभिरक्षणम्
tasmā adāddhariścakraṃ pratyanīkabhayāvaham | ekāntabhaktibhāvena prīto bhṛtyābhirakṣaṇam
SB 9.4.28