Inner Fire Through Korbanos
בפסוק לחמי לאשי ריח ניחוחי תשמרו
On the verse, "My offering, My food for My fire-offerings, a pleasing aroma to Me, shall you safeguard to offer to Me in its appointed time" (Bamidbar 28:2).
The Sfas Emes opens with the verse commanding Bnei Yisrael to safeguard the bringing of the korbanos to Hashem at their proper time.
כי בקבלת התורה דכ' ועל הארץ הראך את אשו זה האש נשאר בבנ"י
For at the giving of the Torah, regarding which it is written, "And upon the earth He showed you His great fire" (Devarim 4:36), this fire remained within Bnei Yisrael.
He teaches that the very fire revealed at Har Sinai at the giving of the Torah did not depart, but remained implanted within Bnei Yisrael themselves.
וזה האש שירד על המזבח מן השמים ובזכות הקרבנות נתקיים זה האש בבנ"י
And this fire, which descended upon the Mizbe'ach from Heaven, was sustained within Bnei Yisrael through the merit of the korbanos.
That same heavenly fire that came down upon the Mizbe'ach is kept alive within Bnei Yisrael specifically through the avodah of bringing korbanos.
ובשבתות ומועדים שיש נשמה יתירה צריכין קרבן מוסף לקיים תוספות התלהבות המתגלה בהם
And on Shabbosos and Yamim Tovim, when there is a neshamah yeseirah, an additional soul, one requires the korban Musaf to sustain the added flame of fervor that is revealed within them.
On Shabbos and Yom Tov, when a person receives a neshamah yeseirah, the extra Musaf offering is needed to sustain the heightened spiritual fervor that those days awaken.
תשמרו להקריב לי לשמי
"Shall you safeguard" — to offer it up to Me, for My sake alone.
He explains the words "shall you safeguard" to mean that the offering must be brought purely for Hashem's sake.
הגם כי בזכות הקרבנות ירד כל השפע בעולם אבל מחשבת בנ"י צריך להיות רק לשמו ית' בלבד
Even though it was through the merit of the korbanos that all the bounty descended into the world, nevertheless the intention of Bnei Yisrael must be directed solely toward His Name, may He be blessed, and to nothing else.
Although the korbanos are the channel through which all blessing flows into the world, Bnei Yisrael must not aim at that benefit; their sole intent must be Hashem Himself.
וכמו כן בתפלות שכנגד תמידין תיקנום צריך להיות עבודה צורך גבוה ואז ממילא מפיקין רצון מהשי"ת:
And likewise with the tefillos, which were instituted to correspond to the daily korbanos, the avodah must be performed for the sake of Heaven, as a need of the One Above, and then, as a matter of course, they draw forth a favorable will from Hashem, may He be blessed.
The same applies to tefillah, which stands in place of the daily korbanos: it must be done as avodah for the sake of Heaven, and only then does it automatically draw down Hashem's favor.
Summary: The Sfas Emes explains that the fire revealed to Bnei Yisrael at the giving of the Torah at Har Sinai was not a passing event, but remained burning within them, and that this same heavenly fire that descended upon the Mizbe'ach is sustained in Bnei Yisrael through the avodah of the korbanos. On Shabbosos and Yamim Tovim, when a neshamah yeseirah brings an added measure of fervor, the Musaf offering is required to sustain that heightened flame. He further teaches, from the words "shall you safeguard," that even though all the bounty of the world flows down through the korbanos, the intention of Bnei Yisrael must be directed solely toward Hashem's Name and nothing else. The same holds true of tefillah, which corresponds to the daily korbanos: when the avodah is done purely as a need of the One Above, it then draws forth Hashem's favorable will of its own accord.