Laudator Temporis Acti

Matthew 6.5-6:

And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward. But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.

But some ancient writers thought that if you prayed in secret or silently, you must be asking for something disgraceful. Here are some examples.

Horace, Epistles 1.16.56-62 (tr. John Conington):

See your good man, who oft as he appears
In court commands all judgments and all ears;
Observe him now, when to the gods he pays
His ox or swine, and listen what he says:
"Great Janus, Phoebus"--this he speaks aloud;
The rest is muttered all and unavowed--
"Divine Laverna, grant me safe disguise;
Let me seem just and upright in men's eyes;
Shed night upon my crimes, a glamour o'er my lies."

vir bonus, omne forum quem spectat et omne tribunal,
quandocumque deos vel porco vel bove placat:
'Iane pater!' clare, clare cum dixit: 'Apollo!'
labra movet, metuens audiri: 'Pulchra Laverna,
da mihi fallere, da iusto sanctoque videri,
noctem peccatis et fraudibus obice nubem.'

Seneca, Letters to Lucilius 10.5 (tr. Richard Gummere):

A statement which I found in Athenodorus is true: "Then know that you are free from all desires when you come to the point that you ask God for nothing except what you could ask for openly." For now how great is the folly of men! They whisper the most shameful prayers to the gods; if someone tries to listen, they fall silent, and they tell to God what they don't want a fellow human to know. Therefore consider whether this advice might not be profitably given: live with men as though God were watching, speak with God as though men were listening.

verum est quod apud Athenodorum inveni: 'tunc scito esse te omnibus cupiditatibus solutum, cum eo perveneris ut nihil deum roges nisi quod rogare possis palam'. nunc enim quanta dementia est hominum! turpissima vota dis insusurrant; si quis admoverit aurem, conticiscent, et quod scire hominem nolunt deo narrant. vide ergo ne hoc praecipi salubriter possit: sic vive cum hominibus tamquam deus videat, sic loquere cum deo tamquam homines audiant.

Seneca, On Benefits 2.1.4 (tr. John Basore):

If men had to make their vows to the gods openly, they would be more sparing of them; so true is it that even to the gods, to whom we most rightly make supplication, we would rather pray in silence and in the secrecy of our hearts.

vota homines parcius facerent, si palam facienda essent ; adeo etiam deos, quibus honestissime supplicamus, tacite malumus et intra nosmetipsos precari.

Seneca, On Benefits 6.38.5 (tr. John Basore):

Lastly, let every man examine himself let him retire into the secrecy of his heart, and discover what it is that he has silently prayed for. How many prayers there are which he blushes to acknowledge, even to himself! How few that we could make in the hearing of a witness!

denique se quisque consulat, et in secretum pectoris sui redeat, et inspiciat quid tacitus optaverit; quam multa sont vota, quae etiam sibi fateri pudet! quam pauca, quae facere coram teste possimus!

Persius 2.3-14 (tr. Lewis Evans):

You at least do not with mercenary prayer ask for what you could not intrust to the gods unless taken aside. But a great proportion of our nobles will make libations with a silent censer. It is not easy for every one to remove from the temples his murmur and low whispers, and live with undisguised prayers. "A sound mind, a good name, integrity"---for these he prays aloud, and so that his neighbor may hear. But in his inmost breast, and beneath his breath, he murmurs thus, "Oh that my uncle would evaporate! what a splendid funeral! and oh that by Hercules' good favor a jar of silver would ring beneath my rake! or, would that I could wipe out my ward, whose heels I tread on as next heir! For he is scrofulous, and swollen with acrid bile. This is the third wife that Nerius is now taking home!"

                          non tu prece poscis emaci
quae nisi seductis nequeas committere divis;
at bona pars procerum tacita libabit acerra.
haut cuivis promptum est murmurque humilisque susurros
tollere de templis et aperto viuere voto.
'mens bona, fama, fides', haec clare et ut audiat hospes;
illa sibi introrsum et sub lingua murmurat: 'o si
ebulliat patruus, praeclarum funus!' et 'o si
sub rastro crepet argenti mihi seria dextro
Hercule! pupillumve utinam, quem proximus heres
inpello, expungam; nam et est scabiosus et acri
bile tumet. Nerio iam tertia conditur uxor.'

Lucan 5.104-105 (tr. J.D. Duff):

There no sinful prayers are framed in stealthy whisper.

haud illic tacito mala voto susurro / concipiunt.

Martial 1.39 (tr. Peter Howell):

If there is anyone who can be counted among those rare friends such as are known to old-fashioned trust and ancient repute; if anyone is steeped in the arts of both Athenian and Latin Minerva, and is truly and genuinely a good man; if anyone is the protector of what is right, an admirer of all that is honourable, and a man who asks nothing of the gods with inaudible voice; if there is anyone who can rely on the strength of a great mind: damn me if it isn't Decianus.

Si quis erit raros inter numerandus amicos,
    quales prisca fides famaque novit anus,
si quis Cecropiae madidus Latiaque Minervae
  artibus et vera simplicitate bonus,
si quis erit recti custos, mirator honesti
  et nihil arcano qui roget ore deos,
si quis erit magnae subnixus robore mentis:
  dispeream si non hic Decianus erit.

I owe these classical Latin parallels to Peter Howell's commentary on Martial (1.39.6). Howell does not, however, cite Matthew 6.5-6.