מדרש על מלאכי 1:1

And God spoke all these words, saying: “I am the Lord thy God” (Exod. 20:1). R. Isaac said: All the prophets received the inspirations for their future prophesies at Mount Sinai. How do we know this to be so? It is written: But with him that standeth here with us this day before the Lord our God, and also with him that is not here with us this day (Deut. 29:14). That standeth here with us this day refers to those who were already born, and with him that is not here alludes to those who were to be born in the future. Hence they are not with us this day. “Not standing here with us this day” is not written in this verse, but rather Is not here with us this day. This alludes to the souls who were to be created in the future, since standing here could not be said of them. They were included in the general statement. And that is why the verse states: The burden of the word of the Lord to Israel by Malachi (Mal. 1:1). It does not say “of Malachi,” but merely by Malachi, indicating that the prophecy had been transmitted to him previously at Sinai. Similarly Isaiah said: Come ye near unto Me, hear ye this: From the beginning I have not spoken in secret; from the time that it was, there am I; and now the Lord hath sent me, and His spirit (Isa. 48:16). Isaiah is saying here that at the time the Torah was given I received the prophecy. Hence it says: from the time that it (the Torah) was, there am I; and now the Lord God hath sent me, and His spirit, but until now He did not give me permission to prophesy.

Another explanation: "And God said all of these things, saying" - Rabbi Yitzchak said, What the prophets were to prophesy in the future in each generation, they received from Mount Sinai. As Moshe said to Israel (Deuteronomy 29:14), "But with those here with us standing today and with those not here with us today." It does not say [at the end of the verse], "with us standing today," but rather, "with us today"; these are the souls that will be created in the future, who do not have substance, about whom "standing" is not mentioned. For even though they did not exist at that time, each one received that which was his. And so [too], it states (Malachi 1:1), "The burden that God spoke to Israel by ['in the hand of'] Malachi" - it does not state, "in the days of Malachi," but rather "in the hand of Malachi," as the prophecy was already in his hand from Mount Sinai, but until that time, he was not given permission to prophesy. Likewise Yeshayahu said (Isaiah 48:16), "from the time it was, was I there." Yeshayahu said, "From the time the Torah was given at Sinai, I was there and received this prophecy, except [only] 'now did God send me and His spirit'" - until now, he was not given permission to prophesy. And it was not only of the prophets who receive their prophecy from Sinai, but also the sages who arise in each generation - each of them received what was his from Sinai. And so [too] it states (Deuteronomy 5:19), "These things did the Lord speak to your entire congregation, [...] a great voice and it did not cease": Rabbi Yochanan said, "One voice was split into seven voices and they were divided into seventy languages"; Rabbi Shimon ben Lakish said, "From it prophesied all of the prophets who arose." The Sages said that it did not have an echo. Rabbi Shmuel bar Nachmani said in the name of Rabbi Yochanan, "What is [the meaning of] 'The voice of the Lord is in strength' (Psalms 29:4)? Can one really say this? Isn't it true that, with even one angel, no creature can stand up to his voice, as it states (Daniel 10:6), 'And its body was like beryl (tarshish) [...] and the voice of its words was like the voice of the multitude.' And does the Holy One, blessed be He, about Whom it is written (Jeremiah 23:24), 'do I not fill the heavens and the earth,' need to speak in strength? Rather, 'The voice of the Lord is in strength' - in the strength of all of the voices." And this verse supports the view of Rabbi Yochanan: "The Lord gives the word, the announcers are a great host" (Psalms 68:12).