Vasiṣṭha's teaching (Mahārāmāyaṇa) 3.60.17

3.60.17

amṛtatvaṃ viṣaṃ yāti sadaivāmṛtavedanāt .
śatrurmitratvamāyāti mitrasaṃvittivedanāt .. 17

Vasishtha said:—
16. [Sanskrit available] Know sinless Rama! that whosoever thinks of anything in any manner at any place or time, he comes to feel the same in the same manner, and in the same place and time.

17. [Sanskrit available] Take for instance the destructive poison, which becomes as ambrosia to venomous insects, that take it for their dainty nourishment; and so is an enemy turning to a friend by your friendly behaviour unto him. (In both cases the evil turns to good by our taking it as such).