“Quis custodiet ipsos custodes? Who will guard the guards?”

VI, line 347
Variant translations:
But who is to guard the guards themselves?
Translated by Lewis Evans, in The Satires of Juvenal, Persius, Sulpicia, and Lucilius (1861), p. 51
Who watches the watchmen?
Famous variant used in the graphic novel, Watchmen by Alan Moore and Dave Gibbons.
The original context is that a husband might lock his wife in the house to prevent her adulteries, but she is cunning and will start with the guards; hence, who guards the guards? The phrase has come to be applied broadly to people or organisations acting against dishonesty or corruption, esp. in public life. See Quis custodiet ipsos custodes? at Wikipedia.
Satires, Satire VI

Thermopylae http://www.cavafy.com/poems/content.asp?id=69&cat=1
Collected Poems (1992)
Context: Honor to those who in the life they lead
define and guard a Thermopylae.
Never betraying what is right,
consistent and just in all they do
but showing pity also, and compassion;
generous when they are rich, and when they are poor,
still generous in small ways,
still helping as much as they can;
always speaking the truth,
yet without hating those who lie.

Source: Going Postal

The Conspiracy of Kings (1792)
Context: Once draw the sword; its burning point shall bring
To thy quick nerves a never-ending sting;
The blood they shed thy weight of wo shall swell,
And their grim ghosts for ever with thee dwell. Learn hence, ye tyrants, ere ye learn too late,
Of all your craft th' inevitable fate.
The hour is come, the world's unclosing eyes
Discern with rapture where its wisdom lies;
From western heav'ns th' inverted Orient springs,
The morn of man, the dreadful night of kings.
Dim, like the day-struck owl, ye grope in light,
No arm for combat, no resource in sight;
If on your guards your lingering hopes repose,
Your guards are men, and men you've made your foes;
If to your rocky ramparts ye repair,
De Launay's fate can tell your fortune there.
No turn, no shift, no courtly arts avail,
Each mask is broken, all illusions fail;
Driv'n to your last retreat of shame and fear,
One counsel waits you, one relief is near :
By worth internal, rise to self-wrought fame,
Your equal rank, your human kindred claim;
'Tis Reason's choice, 'tis Wisdom's final plan,
To drop the monarch and assume the man.

Source: The Bhagavadgītā (1973), p. 75. (10.)

Source: Clockwork Princess

Life Without and Life Within (1859), My Seal-Ring

Speech (3 June 1834); reported in Edward Everett, ed., The Works of Daniel Webster (1851), volume iv, page 47

The Golden Ass (1999)
Context: He sets a thief to guard his purse
Who trusts a dial with his hours
Or bids a sand-glass bleed away his nights,
His days, his loves, his pleasures and his powers.
The burthen of his years
Is Time's soft footfall, Time's soft
Falling
Through his joys and tears.