Midras Daniel 6:21: Mekhilta DeRabbi Yishmael, Sifrei Devarim, Midrash Tehillim & Bereshit Rabbah

And thus with Daniel, viz. (Daniel 6:21) "And when he (the king) approached the den, he cried to Daniel in a mournful voice … (22) (Only) then did Daniel speak with the king, etc."

(Devarim 3:24) "to show Your servant": There are those who call themselves "servants," whom the Holy One Blessed be He calls "servants." And there are those who call themselves "servants," whom the Holy One Blessed be He does not call "servants." And there are those who do not call themselves "servants," whom the Holy One Blessed be He calls "servants." Abraham called himself a "servant," viz. (Bereshith 12:3) "Do not now pass away from Your servant," and the Holy One Blessed be He called him a "servant," viz. (Ibid. 26:24) "for the sake of Abraham, My servant." Jacob called himself a "servant," viz. (Ibid. 32:11) "I am too small for all of the lovingkindnesses and all of the truth that You have done with Your servant," and the Holy One Blessed be He called him a "servant," viz. (Isaiah 41:8) "But you, Israel, are My servant, Jacob, etc." Moses called himself a "servant, viz. "to show Your "servant," and the Holy One Blessed be He called him a "servant," viz. (Joshua 1:2) "Moses My servant is dead." David called himself a "servant," viz. (Psalms 116:16) "Heed, O L-rd, for I am Your servant," and the Holy One Blessed be He called him a "servant," viz. ( II Kings 19:34) "for the sake of David, My servant." Isaiah called himself a "servant," viz. (Isaiah 49:5) "… who formed me from the womb to be a servant to Him," and the Holy One Blessed be He called him a "servant," viz. (Ibid. 20:3) "Isaiah, My servant." Samuel called himself a "servant," but the Holy One Blessed be He did not call him a "servant." Shimshon called himself a "servant," viz. (Judges 15:18) "You have given unto the hand of Your servant, etc.", but the Holy One Blessed be He did not call him a servant. Solomon called himself a "servant," viz. (I Kings 3:9) "Give Your servant an understanding heart," and the Holy One Blessed be He did not call him a "servant," but wrought for his father's sake, viz. (I Kings 15:13) "for the sake of David, My servant." Iyyov did not call himself a "servant," but the Holy One Blessed be He called him a "servant," (viz. Iyyov 2:3) "Have you given heart to My servant, Iyyov"? Joshua did not call himself a "servant," but Scripture called him a "servant," viz. (Joshua 24:29) "and Joshua the son of Nun, the servant of the L-rd, died." Calev did not call himself a "servant," but the Holy One Blessed be He called him a "servant," viz. (Bamidbar 14:24) "But My servant Calev, etc." Elyakim did not call himself a "servant," but the Holy One Blessed be he called him a "servant," viz. (Isaiah 22:20) "And I will call My servant Elyakim, etc." Zerubavel did not call himself a "servant," but the Holy One Blessed be He called him a "servant," viz. (Chaggai 2:23) "Zerubavel ben Shaltiel, My servant." Daniel did not call himself a "servant," but Scripture called him a "servant," viz. (Daniel 6:21) "The king spoke and said to Daniel: Daniel, servant of the living G-d." Chananiah, Mishael, and Azaryah did not call themselves "servants," but the Holy One Blessed be He called them "servants," viz. (Daniel 6:23) "Shadrach, Meshach and Aved-nego, servants of the exalted G-d." The early prophets did not call themselves "servants," but the Holy One Blessed be He called them "servants," viz. (Amos 3:7) " … but He revealed His secret to His servants, the prophets."

Variantly: "and to serve Him": This is prayer. — But perhaps it is service, literally. It is, therefore, written "with all your heart." Now is there "service" in the heart? What, then, is the intent of "and to serve Him with all your heart? Prayer. Similarly, David said (Psalms 141:2) "Let my prayer stand as incense before You; the lifting of my hands, as an evening offering." And it is written (Daniel 6:11) "And when Daniel learned that the writing had been inscribed, he went home. He had windows open in his upper storey, facing Jerusalem, and three times a day he fell to his knees and gave thanks before his G-d, exactly as he used to do before this." And (Ibid. 21) "And when he (the king) drew near to the pit, to Daniel, he cried out in a sad voice. The king answered and said to Daniel: 'Daniel, servant of the living G-d, was your G-d, whom you serve continually, able to save you from the lions?'" Now was there (Temple) service in Bavel? What, then, is the intent of "and to serve Him"? Prayer. Just as the sacrificial service is called "service," so, prayer.