Shatapatha Brahmana 9.1.1.6

9.1.1.6

yadvevaitacatarudriyaṃ juhoti | prajāpatervisrastāddevatā udakrāmaṃstameka eva devo nājahānmanyureva so 'tinnantarvitato 'tiṣṭhatso 'rodīttasya yānyaśrūṇi prāskandaṃstānyasminmanyau pratyatiṣṭhantsa eva śataśīrṣā rudraḥ samabhavatsahasrākṣaḥ śateṣudhiratha yā anyā vipruṣo 'pataṃstā asaṃkhyātā sahasrāṇīmāṃlokānanuprāviśaṃstadyadruditātsamadrudrāḥ so 'yaṃ śataśīrṣā rudraḥ sahasrākṣaḥ śateṣudhiradhijyadhanvā pratihitāyī bhīṣayamāṇo 'tiṣṭhadannamicamānastasmāddevā abibhayuḥ

And as to why he performs the Satarudriya offering. When Pragâpati had become disjointed, the deities departed from him. Only one god did not leave him, to wit, Manyu (wrath): extended he remained within. He (Pragâpati) cried, and the tears of him that fell down settled on Manyu. He became the hundred-headed, thousand-eyed, hundred-quivered Rudra. And the other drops that fell down, spread over these worlds in countless numbers, by thousands; and inasmuch as they originated from crying (rud), they were called Rudras (roarers). That hundred-headed, thousand-eyed, hundred-quivered Rudra, with his bow strung, and his arrow fitted to the string, was inspiring fear, being in quest of food. The gods were afraid of him.