Shiva Purana
जगतः पितृभूतानां शिवो मूर्त्यात्मनामपि ॥ पितृभावेन सर्वेषां पितामह उदीरितः ॥ ३८ ॥
Śiva is the progenitor of the souls that have attained the status of fathers. He is therefore called the grandfather.
english translation
jagataH pitRbhUtAnAM zivo mUrtyAtmanAmapi ॥ pitRbhAvena sarveSAM pitAmaha udIritaH ॥ 38 ॥
hk transliteration by Sanscript