“The greatest reward of righteousness is peace of mind.”

Maxim 519, trans. Stopp
Maxims and Reflections (1833)

Entering Your Destiny (2020)

Variant: You’re the greatest risk I’ve ever taken.” His pressed his lips gently to mine. “And the greatest reward.
Source: Reflected in You

"Hymn for Christmas-Day"
Hymns and Sacred Poems (1739)
Context: Hail the heavenly Prince of Peace!
Hail the Sun of Righteousness!
Light and life to all he brings,
Risen with healing in his wings.
Mild he lays his glory by,
Born that man no more may die,
Born to raise the sons of earth,
Born to give them second birth.

1900s, Inaugural Address (1905)
Context: While ever careful to refrain from wrongdoing others, we must be no less insistent that we are not wronged ourselves. We wish peace, but we wish the peace of justice, the peace of righteousness. We wish it because we think it is right and not because we are afraid. No weak nation that acts manfully and justly should ever have cause to fear us, and no strong power should ever be able to single us out as a subject for insolent aggression.

Saying 141

On receiving Congressional Medal for Distinguished Civilian Achievement (23 April 1956); several variations of this personal motto are often quoted, including:
The reward for work well done is the opportunity to do more.
As quoted in 50 American Heroes Every Kid Should Meet! (2001) by Dennis Denenberg and Lorraine Roscoe, p. 99
I feel that the greatest reward for success is the opportunity to do more.

1910s, Nobel lecture (1910)
Context: In our complex industrial civilization of today the peace of righteousness and justice, the only kind of peace worth having, is at least as necessary in the industrial world as it is among nations. There is at least as much need to curb the cruel greed and arrogance of part of the world of capital, to curb the cruel greed and violence of part of the world of labor, as to check a cruel and unhealthy militarism in international relationships.

Speech in the Circus, Anlaby Road, Hull (8 March 1899), quoted in The Times (9 March 1899), p. 6
Leader of the Opposition