Rudra-prasava-varṇana
The Manifestation and Naming of Rudra / Nīlalohita
विवेश तच्चन्द्रमसं महादेवस्ततः शशी / तस्माद्विभाव्यते ह्येष महादेवस्तु चन्द्रमाः
viveśa taccandramasaṃ mahādevastataḥ śaśī / tasmādvibhāvyate hyeṣa mahādevastu candramāḥ
Mahādeva entered that orb of the Moon; then it became Śaśī. Therefore this Moon is understood to be Mahādeva himself.