It is not yours to finish the task, but neither are you free to set it aside.

לֹא עָלֶיךָ הַמְּלָאכָה לִגְמֹר, וְלֹא אַתָּה בֶן חוֹרִין לִבָּטֵל מִמֶּנָּה.

The Talmud (AD 200-500) Collection of Jewish rabbinical writings

Mishnah, Pirkei Avot [Ethics of the Fathers; פִּרְקֵי אָבוֹת] 2:16
    (Source)

In some editions cited as 2:15 or 2:21. Many of the references below ("E.g.") have no identified translator, nor is it clear whether the translations are from the surrounding writers or borrowed from elsewhere.

Quoting Rabbi Tarfon (c. AD 130). While literally speaking of studying the Torah, the passage is usually understood to imply solving the problems of the world.

(Source (Hebrew)). Other translations:

It is not for thee to finish the work, nor art thou free to desist therefrom.
[tr. Taylor (1897)]

It is not incumbent upon you to complete the work, but neither are you at liberty to desist from it.
[tr. Charles (1913); 2:21]

The work is not upon thee to finish nor art thou free to desist from it.
[tr. Herford (1929); 2:21]

It is not thy part to finish the task, yet thou art not free to desist from it.
[tr. Danby (1933)]

It is not up to you to complete the work; but neither are you free to desist from it.
[tr. Bokser (1989)]

It is not your responsibility to finish the work [of perfecting the world], but you are not free to desist from it either.
[tr. Telushkin (1991)?]

It is not your duty to finish the work, but neither are you at liberty to neglect it.
[tr. Kulp (2014?)]

The work is not yours to complete. [The Holy One Blessed be He did not hire you to complete all of it, in which instance you would lose your wage if you did not complete it.] And [lest you say: (In that case,) I will not learn and I will not take a wage] — you are not free to abstain from it. [Perforce the yoke is upon you to labor.]
[tr. Silverstein (2013?)]

It is not [incumbent] upon thee to finish the work, but neither art thou a free man so as to [be entitled to] refrain therefrom.
[Wikisource]

It is not your responsibility to finish the work, but neither are you free to desist from it.
[tr. Open Mishnah]

It is not your duty to finish the work, but neither are you free to desist from it.
[E.g.]

It is not incumbent upon you to finish the task, but neither are you free to absolve yourself from it.
[E.g.]

It is not up to you to finish the task, but you are not free to avoid it.
[E.g.]

You need not finish the work , but you are not free to stop working.
[E.g.]

Although I am not free to avoid doing the work, it is not always necessary that I finish the task.
[E.g.]

You are not obliged to complete the task, nor are you free to abandon it.
[E.g.]

You are not obligated to complete the work, but neither are you free to abandon it.
[E.g.]

It is not your duty to complete the work, but neither are you free to desist from it.
[E.g.]

It is not your obligation to finish the task, nor are you free to desist from it.
[E.g.]

It is not yours to finish the task, but neither are you free to desist from it.
[E.g.]