Mbh 5.10.4a

“Salya said, ‘Hearing these words of the great Rishis, the illustrious Asura spoke to them, “No doubt, the Rishis, endued with supernatural powers, are to be respected by me. Let what I am going to say, ye gods, be performed in its entirety; then I shall do everything that (these) best of Brahmanas have said to me. Ye lords of the Brahmana race, ordain so that Indra himself or the gods do not kill me by what is dry, or wet; by stone, or by wood; by a weapon fit for close fight, or by a missile; in the day time, or at night. On those terms eternal peace with Indra would be acceptable to me,”--Very good! was what the Rishis told him, O best of Bharata race. Thus peace having been concluded, Vritra was very much pleased. And Indra also became pleased though constantly occupied with the thought of killing Vritra. And the chief of the deities passed his time in search of a loophole, uneasy (in mind). And on a certain day when it was evening and the hour awful, Indra caught sight of the mighty Asura on the coast of the sea. And he bethought himself of the boon that was granted to the illustrious Asura, saying, “This is the awful evening time; it is neither day, nor night; and this Vritra, my enemy, who hath stripped me of my all, must undoubtedly be killed by me. If I do not kill Vritra, this great and mighty Asura of gigantic frame, even by deceit, it will not go well with me.” And as Indra thought of all this, bearing Vishnu in mind he beheld at that instant in the sea a mass of froth as large as a hill. And he said, “This is neither dry, nor wet, nor is it a weapon; let me hurl it at Vritra. Without doubt, he will die immediately.” And he threw at Vritra that mass of froth blended with the thunderbolt. And Vishnu, having entered within that froth, put an end to the life of Vritra. And when Vritra was killed, the cardinal points were free from gloom; and there also blew a pleasant breeze; and all beings were much pleased.