Erasmus, Adagia 152

It is proper to remember the living. This is an old adage directed at those who talk excessively about the dead. It is commonly thought to be a bad omen to have the dead one the tip of one’s tongue and to bring them forth and cite them in speech. For this reason M. Varro in Book 3 of On the Latin Language thinks that letum (death) is derived ἀπὸ τῆς λήθης, that is from forgetting, as if to suggest that it is proper for one to move into oblivion once they have died. In funerals, it was once customary to hear from the announcet: “He has been given to death.” 

The adage is related by Cicero in his On the Ends of Good and Evil. There, when Piso and then Quintus Cicero have said that they are violently moved by the recollection of famous men from the contemplation of the places in which they had at some time been when they were alive, and when each of them decided by whose memory they were delighted the most, Pomponius Atticus added as if joking, ‘But I, whom you are accustomed to upbraiding as totally given over to Epicurus, am much more in the camp of Phaedrus, whom I esteem foremost, as you know, in the gardens of Epicurus, which we were just passing, but I remember the living in accordance with the advice of the ancient proverb: yet I could not forget Epicurus if I wanted to, since our closest friends have his image not just on tablets, but even on their cups and rings.’ So much for Cicero.

Similarly, Plautus in his Truculentus: 

You may know a person while they live; once they are dead, you should remain silent.

But now the rabble doesn’t even remember the good deeds of friends, since this saying of Thales is rightly celebrated, that it is proper to be mindful no less of absent friends than of present ones.

Vivorum meminisse oportet. Vetus adagium in eos, qui plurimum de vita defunctis loquuntur, id quod vulgo putant ominosum mortuos in ore habere eosque ut velut citatos in sermonem adducere. Unde et M. Varro libro De lingua Latina tertio putat lethum ἀπὸ τῆς λήθης, id est oblivione dictum, quasi in oblivionem abire conveniat, qui vita excesserit, atque in funeribus sic quondam a praecone dici solere: Ollus letho datus est. Refertur adagium a Cicerone libro quinto De finibus bonorum et malorum. Ubi cum Piso, deinde Q. Cicero dixissent se vehementer commoveri recordatione clarorum virorum ex contemplatione locorum in quibus aliquando vivi versati fuissent, et uterque recensuisset, quorum memoria potissimum delectaretur, tum Pomponius Atticus quasi iocans: At ego, inquit, quem vos ut deditum Epicuro insectari soletis, sum multum equidem cum Phaedro, quem unice diligo, ut scitis, in Epicuri hortis, quos modo praeteribamus, sed veteris proverbii admonitu vivorum memini; nec tamen Epicuri licet oblivisci, si cupiam, cuius imaginem non modo in tabulis nostri familiares, sed etiam in poculis et anulis habent. Hactenus Cicero. Plautus item in Truculento:

Dum vivit, hominem noveris; dum mortuus est, quiescas.

At nunc vulgus ne beneficium quidem amicorum meminit, cum Thaletis dictum iure celebretur oportere non minus absentium amicorum quam praesentium memores esse.