The sentiment on this stone sounds Biblical in origin, but I haven’t yet been able to find this phrase. I am usually able to discern Bible quotes simply by entering them into an internet search, but I’m not always as successful with locating quotes from religious poetry or hymns. If someone out there is more knowledgeable than me on the source for this epitaph, I look forward to hearing from them.
There is the possibility, that, someone edits a statement for his own purpose. This has been done in business, politics, and public speaking (preachers amongst others). Sometimes it is merely a mistaken phrasing. It may even be a quotation the deceased had made.
There may be a close scriptural allusion: “and as thy days, so shall thy strength be.” –Deuteronomy xxxiii. 25b (KJV)