The true reward is bittul to Hashem

אשר יעשה אותם כו' וחי בהם.

"Which a man shall do, and live by them" (Vayikra 18:5).

The Sefas Emes opens with the pasuk that promises "vachai bahem" — that one lives through the mitzvos — which he will interpret as a description of the true reward of a mitzvah.

אם כי אחז"ל אל תהיו כעבדים המשמשים ע"מ לקבל פרס.

Although Chazal said, "Do not be like servants who serve in order to receive a reward (pras)" (Avos 1:3).

He notes the apparent tension: the Mishnah warns against serving Hashem for the sake of reward, which seems to clash with the pasuk's promise of "living" through mitzvos.

אך אמרו ג"כ הוי מחשב הפסד מצוה כו'.

Yet they also said, "Calculate the loss [against the gain] of a mitzvah..." (Avos 2:1).

On the other hand, Chazal do tell us to weigh the "loss against the reward" of a mitzvah — implying that there is a legitimate concept of reward to consider. He will reconcile the two.

כי בוודאי שכר עוה"ב אינו ענין הגוף.

For certainly the reward of the World to Come is not a matter of the body.

The reward of Olam Haba is not physical pleasure or bodily gain; it is something of an entirely different order.

רק זה עצמו עוה"ב מה שזוכין להיות בטל להש"י כראוי.

Rather, this itself is the World to Come — that one merits to be nullified to Hashem as is fitting.

The very essence of Olam Haba is the state of bittul: meriting to be completely nullified before Hashem. The reward is not separate from the avodah — it is its fulfillment.

שהוא דבר שא"י להשיג ולהיות ברור שאין להשיג לרוב ההעלם לכך נק' עולם כמו שעולם הזה נק' העלם על שהחושך מכסה ומעלים כבודו ית'.

Which is something one cannot grasp, and it is clear that it cannot be grasped because of the great concealment — therefore it is called "olam" (world), just as this world is called "he'elem" (concealment), since the darkness covers and conceals His glory.

Playing on the words olam and he'elem (concealment), he explains that both worlds are named for hiddenness. This world is "he'elem" because darkness conceals Hashem's kavod; the term applies to both realms, each a form of concealment.

כמו כן עוה"ב הוא העלם של אמת על שאין יכולין להשיג.

So too the World to Come is a concealment of truth, since one cannot grasp it.

Olam Haba is also a "concealment" — but a concealment of truth: it is hidden from us not because of falsehood but because it utterly transcends human comprehension.

ובעוה"ז ההעלם ע"י השקר.

And in this world the concealment is by way of falsehood.

The two concealments differ in their cause: this world hides Hashem through sheker (falsehood and illusion), whereas the World to Come is hidden only because it is too true and exalted to be grasped.

ובוודאי כן חשבון הפסד מצוה מה שע"י המצוה מדבק האדם עצמו בהש"י שהוא שורש החיות.

And certainly this is the "calculation of the loss of a mitzvah" — that through the mitzvah a person attaches himself to Hashem, who is the root of all vitality.

The "reckoning" of a mitzvah's worth is precisely this: through performing it, a person achieves deveikus, binding himself to Hashem, the shoresh hachiyus (root of all life). That is the gain to be weighed.

וכן וחי בהם כנ"ל זה השכר הוא תכלית עשיות המצות.

And likewise "and live by them," as above — this is the reward, the very purpose of performing the mitzvos.

"Vachai bahem" means that the mitzvah connects one to the Source of life; this "living" is itself the reward and the entire purpose of doing mitzvos — not a separate prize.

ולקבל פרס הוא לשון פרוסה שכוונתו לעצמו ואינו דבק בשורש החיות כנ"ל היפוך וחי בהם כנ"ל:

And "to receive a reward (pras)" is from the word "perusah" (a slice/fragment), for his intent is for himself, and he is not attached to the root of vitality — the opposite of "and live by them," as above.

The Sefas Emes plays on pras / perusah (a broken-off slice): one who serves for "reward" wants a detached fragment for his own self-interest, cutting himself off from the Source. That is the very opposite of "vachai bahem," which is about cleaving to the root of life itself.

Summary: The Sefas Emes reconciles Chazal's warning against serving for reward with the promise of "and live by them." The true reward of the World to Come is not bodily, but the state of bittul — being nullified to Hashem and cleaving to Him, the root of all life. Both worlds are realms of concealment (olam / he'elem), but this world hides Hashem through falsehood while Olam Haba is hidden only because its truth surpasses comprehension. Serving "for reward" (pras, a self-serving fragment) is the opposite of "vachai bahem," which means attaching oneself to the Source of life.