“Who will guard the guards? If we're the guards of society, then who will watch us and make sure that we're not dangerous?”

2015, Eulogy for the Honorable Reverend Clementa Pinckney (June 2015)
Context: But I don't think God wants us to stop there. For too long, we’ve been blind to the way past injustices continue to shape the present. Perhaps we see that now. Perhaps this tragedy causes us to ask some tough questions about how we can permit so many of our children to languish in poverty, or attend dilapidated schools, or grow up without prospects for a job or for a career. Perhaps it causes us to examine what we’re doing to cause some of our children to hate. Perhaps it softens hearts towards those lost young men, tens and tens of thousands caught up in the criminal justice system and leads us to make sure that that system is not infected with bias; that we embrace changes in how we train and equip our police so that the bonds of trust between law enforcement and the communities they serve make us all safer and more secure. Maybe we now realize the way racial bias can infect us even when we don't realize it, so that we're guarding against not just racial slurs, but we're also guarding against the subtle impulse to call Johnny back for a job interview but not Jamal. So that we search our hearts when we consider laws to make it harder for some of our fellow citizens to vote. By recognizing our common humanity by treating every child as important, regardless of the color of their skin or the station into which they were born, and to do what’s necessary to make opportunity real for every American -- by doing that, we express God’s grace.

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.

The Glass Bead Game (1943)

Source: Going Postal

The Army Times (6 September 2004) http://www.armytimes.com/story.php?f=1-ARMYPAPER-323322.php

Source: The Beloved Returns (1939), Ch. 7
Context: Hold fast the time! Guard it, watch over it, every hour, every minute! Unregarded it slips away, like a lizard, smooth, slippery, faithless, a pixy wife. Hold every moment sacred. Give each clarity and meaning, each the weight of thine awareness, each its true and due fulfillment.

Source: The Tenant of Wildfell Hall (1848), Ch. III : A Controversy; Helen to Gilbert