Anushasana Parva 85.76

To a sacrifice performed formerly by that foremost of gods, viz., the Lord of Rudra, O you of great power, who on that occasion had assumed the form of Varuna, there came the ascetics and all the celestials headed by Agni. There also came to that sacrifice all the sacrificial limbs (in their embodied forms), and the Mantra called Vashat in his embodied forin.