In the Vana Parva of the Mahabharata, the sun god Surya warns his son Karna that Indra will come in disguise and ask for his armor and earrings. “When the wielder of the thunder-bolt asks you for them, do you never give him your ear-rings!”
Karna refuses the advice. “Permit me now, O foremost of celestials, to observe my vow,” he answers, “so that unto him of the thunderbolt coming to beg of me, I may give away even my life!”