Midrasch zu Mischlej 25:13

He sent forth the raven to ascertain what was (the state of) the world. It went and found a carcase of a man cast upon the summit of a mountain, and it settled thereon for its food, and it did not return with its message to its sender, as it is said, "And he sent forth the raven" (Gen. 8:7). He sent forth the dove to see what was (the state of) the world, and she brought back her message to her sender, as it is said, "And the dove came in to him at eventide, and, lo, in her mouth an olive leaf pluckt off" (Gen. 8:11). And why in her mouth was an olive leaf pluckt off? || The dove spake before the Holy One, blessed be He, saying: Sovereign of all worlds ! Let my food be bitter like this olive, and let it be entrusted to Thy hand, and let it not be sweet (even) as honey, and given by the hand of flesh and blood. Hence they said: He who sends a message by the hand of an unclean (messenger) is (like) sending by the hand of a fool, and he who sends a message by the hands of a clean (messenger) is like sending by the hand of a messenger faithful to his senders.

It is only written here (in the text concerning) Moses (that) "he drew near." The rest of the commandments He spake through the mouth of Moses, and concerning him the text says, "As the cold of snow in the time of harvest, so is a faithful messenger to them that send him" (Prov. 25:13).