Smiles are contagious; so are tears; to see
Another sobbing, brings a sob from me.
No, no, good Peleus; set the example, pray,
And weep yourself; then weep perhaps I may.

[Ut ridentibus adrident, ita flentibus adflent
humani voltus. Si vis me flere, dolendum est
primum ipsi tibi: tum tua me infortunia laedent,
Telephe vel Peleu.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Epistles [Epistularum, Letters], Book 2, ep. 3 “Art of Poetry [Ars Poetica; To the Pisos],” l. 101ff (2.3.101-103) (19 BC) [tr. Conington (1874)]
    (Source)

Telephus and Peleus were mythic figures in well-known Greek tragedies. The advice is offered up to those who write of or act/declaim the roles of such characters.

(Source (Latin)). Other translations:

The cheares of men as these will smerke on those that use to smyle:
So are theye wrinchd, when theye do weepe and chaungd within a whyle.
If thou wouldste have me weepe for the firste muste thou pensyfe be.
Thy harmes shall hitte me, when I spy that they have harmed thee.
[tr. Drant (1567)]

To worke the hearers minds, still to the plight.
Mens count'nances, with such as laugh, are prone
To laughter: so they grieve with those that mone:
If thou wouldst have mee weep, bee thou first dround
Thy selfe in tears, then me thy harms will wound,
[tr. Jonson (1640); l. 145ff]

We Weep and Laugh as we see others doe,
He only makes me sad who shews the way,
And first is sad himself, then (Telephus)
I feel the weight of your Calamities,
And fancy all your miseries my Own.
[tr. Roscommon (1680)]

With them, who laugh, our social joy appears;
With them, who mourn, we sympathise in tears;
If you would have me weep, begin the strain,
Then I shall feel your sorrows, feel your pain.
[tr. Francis (1747)]

With those that smile, our face in smiles appears;
With those that weep, our cheeks are bath'd in tears:
To make me grieve, be first your anguish shown,
And I shall feel your sorrows like my own.
[tr. Coleman (1783)]

From face to face as smiles contagious creep,
so weeps the according eye with those that weep.
Who claims my tears, must first display his own;
Then shall I catch his pangs and share his moan.
[tr. Howes (1845)]

As the human countenance smiles on those that smile, so does it sympathize with those that weep. If you would have me weep you must first express the passion of grief yourself; then, Telephus or Peleus, your misfortunes hurt me.
[tr. Smart/Buckley (1853)]

A face all smiles makes other faces smile,
A face all tears will tears from others wile.
Unless, then, in your voice a sob I hear,
You will not wring from me a single tear.
[tr. Martin (1881)]

As human countenances laugh with those who laugh so they weep with those who weep. If you desire me to weep, O Telephus or Peleus, yourself must first lead the way; then you thrill through me.
[tr. Elgood (1893)]

As men's faces smile on those who smile, so they respond to those who weep. If you would have me weep, you must first feel grief yourself: then, O Telephus or Peleus, will your misfortunes hurt me.
[tr. Fairclough (Loeb) (1926)]

As the human face answers a smile with a smile, so does it wait upon tears; if you would have me weep, you must first of all feel grief yourself; then and not till then will your misfortunes, Telephus or Peleus, touch me.
[tr. Blakeney; ed. Kramer, Jr. (1936)]

A man’s face is wreathed in smiles when he sees someone smile;
It twists when he sees someone cry; if you expect me
To burst into tears, you have to feel sorrow yourself.
Then your woes will fasten on me, O Telephus, Peleus.
[tr. Palmer Bovie (1959)]

Just as laughter inspires laughter, tears bring tears
to human faces; if you want my tears, you first must
weep yourself. Then your agonies will hurt me too.
[tr. Fuchs (1977)]

We smile when we see smiling, weep at tears:
Ask me to sob
when you can sob
yourself --
Then (ah) tragic heroes are tragic
(To me).
[tr. Raffel (1983 ed.)]

Men smile if the language smiles;
They weep if the language truly weeps. If you
Desire to hear me weep, you must truly grieve,
O Peleus or Telephus, and I
Grieve as if I suffered your cause of grief.
[tr. Ferry (2001)]

When a person smiles, people's faces smile in return;
when he weeps, they show concern. Before you can move me to tears,
you must grieve yourself. Only then will your woes distress me.
[tr. Rudd (2005 ed.)]

As the human face smiles at a smile, so it echoes
Those who weep: if you want to move me to tears
You must first grieve yourself: then Peleus or Telephus
Your troubles might pain me.
[tr. Kline (2015)]