Ramayana
पुन्नाम्नो नरकाद् यस्मात् पितरं त्रायते सुतः । तस्मात् पुत्र इति प्रोक्तः पितॄन् यः पाति सर्वतः ॥ २-१०७-१२
Since a son delivers his father from a place of torment (hell) called 'Put', he is named as 'Putra'- 'he who delivers his ancestors from all dangers'. ॥ 2-107-12॥
english translation
punnAmno narakAd yasmAt pitaraM trAyate sutaH । tasmAt putra iti proktaH pitRRn yaH pAti sarvataH ॥ 2-107-12
hk transliteration by Sanscript