Vishnu Purana — Amsha 4 — Chapter 20

ViP 4 20.5

यं यं कराभ्यां स्पृशति जीर्णं यौवनम् एति सः शान्तिं चाप्नोति येनाग्र्यां कर्मणा तेन शांतनुः ॥

yaṃ yaṃ karābhyāṃ spṛśati jīrṇaṃ yauvanam eti saḥ śāntiṃ cāpnoti yenāgryāṃ karmaṇā tena śāṃtanuḥ //

Whomever, being aged, he touched with his two hands attained youth, and obtained peace; by that foremost deed he (was called) Śāṃtanu.

AI translation rendered from the Sanskrit, not a published edition