Petrarch, Secretum 3.17:

Don’t let the abundance of days and the contrived distinction of the age fool you. All of human life, however much it may be extended, is like a single day, and not even a whole one. You should frequently call back to mind Aristotle’s comparison, which I noticed pleases you a lot, and which you can scarcely read or hear without it making a real impression on your mind. You can find it put in more sparkling language (and certainly more apt to persuade) in Cicero’s Tusculan Disputations: ‘Aristotle says that certain little beasts which live for only one day are born near the Hypanis, which flows from part of Europe into the Black Sea. One of these who dies at sunrise dies as a youth; one who dies at noon has already achieved an advanced age; but one who departs at the setting of the sun dies old, especially if it is the solstice. Compare the entirety of our life with eternity, and we will be found to exist for just as short a time as that animal.’

This claim seems to me so true that it was diffused from the mouths of philosophers into common use. For have you not seen that even rude and ignorant people have drawn it into their daily parlance, as when they see a boy and say, ‘The sun rises for him,’ then see a man and say, ‘He’s reaching noon, and that guy is at his ninth hour,’ and when they see an old person, ‘That guy has come all the way to the evening and sunset of his life.’ I would then, my dear boy, having you revolve these things in your mind along with anything else of the sort which occurs to you (and I have no doubt that there is much of it). But these are the things which have obtruded themselves upon me at the moment. I beg you for one thing further. Contemplate with more diligence the graves of the ancients, but of those who lived with you, in the certainty that the very same seat and eternal reception hall will be prepared for you. All of us bend our course that way. This is the final home for everyone.

Nec te fallat dierum pluralitas et etatis operosa distinctio: tota hominum vita, quantumlibet extendatur, diei unius instar habet, eiusque vix integri. Crebro ante oculos revoca aristotelicam quandam similitudinem, quam animadverti tibi admodum placere, vixque unquam sine gravi mentis impulsu legi solere vel audiri; quam clariori eloquio et ad persuadendum aptiori in Tusculano quidem a Cicerone relatam invenies, aut his verbis aut profecto similibus, neque enim libri nunc illius copia est “Apud Hypanim” inquit “fluvium, qui ab Europe parte in Pontum influit, bestiolas quasdam nasci scribit Aristotiles, que unum diem vivant; harum que oriente sole moritur, iuvenis moritur; que vero sub meridie, iam etate provectior, at que sole occidente senex abit, eoque magis si solstitiali die. Confer universam etatem nostram cum eternitate, in eadem propemodum brevitate reperiemur ac ille”. Que quidem assertio meo iudicio tam vera est, ut ex ore philosophorum iampridem in vulgus diffusa sit. Nunquid enim rudes etiam et ignaros homines in quotidiani sermonis usum deduxisse vides, ut puerum aspicientes dicant: “Huic sol oritur”, virum autem: “Hic meridiem attigit; hic nonam”; senem vero decrepitum: “Ad vesperam atque ad solis occasum iste pervenit”. Hec igitur, fili carissime, tecum volve et, siqua huius generis occurrunt alia, que multa esse non dubito; sed hec erant que ex tempore se se obtulerunt. Unum preterea obsecro. Sepulcra veterum, sed eorum qui tecum vixere, diligentius, contemplare, certus eandem tibi sedem ac perennem aulam fore preparatam. Tendimus huc omnes. Hec est domus ultima cuntis.