Do Justly, Now
Do not be daunted
by the enormity
of the world's grief.
Do justly, now.
Love mercy, now.
Walk humbly, now.
You are not obligated
to complete the work,
but neither are you free
to abandon it.
Commonly attributed to The Talmud. Attributed here to Annesley William Streane (1844-1915), a Cambridge scholar and Old Testament translator and commentator.
This source attributes the quote to Rabbi Tarfon, in the Pirkei Avot, commenting on Micah 6:8. See also this reddit thread. So I suppose the quote must be from Streane's translation of Tarfon's commentary.
Either way, it's a beautiful passage.