Shanti Parva 341.13

Within that lotus was born Brahman, originating from Aniruddha's grace. Towards the evening of Brahman's day, Aniruddha became filled with anger, and as a consequence of this, there originated from his forehead a son called Rudra gifted with the power of destroying everything. These two, viz., Brahman and Rudra, are the foremost of all the gods, having originated respectively from the Cheerfulness and the Anger (of Aniruddha).