בֵּאדַ֜יִן מַלְכָּ֣א אֲמַ֗ר וְהַיְתִיו֙ לְדָ֣נִיֵּ֔אל וּרְמ֕וֹ לְגֻבָּ֖א דִּ֣י אַרְיָוָתָ֑א עָנֵ֤ה מַלְכָּא֙ וְאָמַ֣ר לְדָנִיֵּ֔אל אֱלָהָ֗ךְ דִּ֣י אנתה [אַ֤נְתְּ] פָּֽלַֽח־לֵהּ֙ בִּתְדִירָ֔א ה֖וּא יְשֵׁיזְבִנָּֽךְ׃
이에 왕이 명하매 다니엘을 끌어다가 사자굴에 던져 넣는지라 왕이 다니엘에게 일러 가로되 너의 항상 섬기는 네 하나님이 너를 구원하시리라 하니라
Rabbeinu Bahya
ולעבדו בכל לבבכם, “and to serve Him with all your hearts.” Our sages in Taanit 2 ask rhetorically: “what kind of service can one perform for G’d with one’s heart?” They answer: “Prayer.” They prove this from Daniel 6,17: “your G’d, the one whom you serve constantly, He will save you.” Was there then a Temple for the Jewish G’d in Babylon where Daniel could have offered sacrifices? Clearly the King must have referred to the G’d to whom Daniel prayed in his heart. In verse 11 of that same chapter Daniel is described as opening the window of his room facing Jerusalem when praying. David also said in Psalms 141,2: “take my prayer as an offering of incense.” Moreover, our sages in Berachot 26 have said that the daily prayers nowadays are in lieu of the daily communal offerings, the תמידים.
Rashi
ולעבדו בכל לבבכם AND TO SERVE HIM WITH ALL YOUR HEART — i.e. to serve Him with a service that is in the heart: that is prayer, for prayer is termed service (עבודה), as it is said, (Daniel 6:17) “Thy God whom thou servest continually”. But was there service (i.e. sacrificial service, the technical term for which is עבודה) in Babylon? But the term is used because he offered prayer there, as it is said, (Daniel 6:11) “Now his windows were open [in his upper chamber towards Jerusalem and he kneeled upon his knees three times a day and prayed]”. And so, too, it states in the case of David (Psalms 141:2): “Let my prayer be set forth as incense before Thee” (incense being part of the sacrificial service) (Sifrei Devarim 41:25-27).