Ramayana
पूर्वं सौहृदबद्धो ऽस्मि वृत्रस्य ह महात्मनः । तेन युष्मत्प्रियार्थं हि नाहं हन्मि महासुरम् ॥ 7-85-4 ॥
"In former times I was bound by friendship to the great-souled Vritra; therefore, for your sake, I do not slay that mighty demon."
english translation