Shiva Purana
सनत्कुमारं च ऋषिं सर्वेषामपि पूर्वजम् ॥ सप्त चैते प्रजायंते पश्चाद्रुद्राश्च सर्वतः ॥ १८ ॥
He created the sage Sanatkumāra too, the eldest of all. They are thus seven born. The Rudras were born afterwards.
english translation
sanatkumAraM ca RSiM sarveSAmapi pUrvajam ॥ sapta caite prajAyaMte pazcAdrudrAzca sarvataH ॥ 18 ॥
hk transliteration by Sanscript