Ramayana
तं राक्षसात्मजं चक्रे मातुरेव वयःसमम् । अमरं चैव तं कृत्वा महादेवो ऽक्षरो ऽव्ययः ॥ 7-4-29 ॥
the great god Mahadeva, the imperishable and unchanging one, made that son of the Rakshasa equal in age to his very mother, and moreover rendered him immortal.
english translation