Exode 3:18

And they will hearken to thee: and thou and the elders of Israel shall go to the king of Mizraim and say to him, The Lord God of the Jehudaee hath called us; and now let us go a journey of three days into the wilderness, to sacrifice before the Lord our God.

AND THEY SHALL HEARKEN TO THY VOICE. “I.e., of their own accord. As soon as you will mention to them this expression, [namely, the double use of the word pakod — pakod pakad’ti (I have surely remembered), mentioned in Verse 16], they will immediately hearken to your voice for they have long had this sign as a tradition from Jacob and Joseph, that by use of this expression they will be delivered [from Egypt]. Jacob said to them, and G-d will surely remember you, and Joseph said to them, G-d will surely remember you.”, Verse 25. Thus the language of Rashi. Perhaps Rashi will explain that Joseph used this expression twice in order to affirm that it was a tradition he received from his father.
And in Eileh Shemoth Rabbah, the Rabbis said: “[As soon as you will say to them pakod pakad’ti], they will immediately hearken to your voice. Why? It is because they have a tradition of the words [wherein G-d will announce the approaching redemption], so that the redeemer who will come and mention to them this double p’kidah [remembrance] is the true redeemer.” Thus the language of the Rabbis on this Agadah.
Now you may ask: Why should they listen to Moses? Perhaps he heard this tradition as well as they did. In reply it may be said that thus they received the tradition from Joseph, who heard it from their father the prophet: the first person who will come and announce the message with this expression [pakod pakad’ti] will be the one who will deliver them [from Egypt], it being revealed and known before the Holy One, blessed be He, that no man will come and deceive them. This He promised them.
But in another place in the Midrash I found:, 5:1. “Said Rabbi Chama the son of Rabbi Chanina: ‘When Moses was in his twelfth year, he was torn up from his father’s house. Why? Had he grown up in his father’s house, the children of Israel would not have believed in him when he came and told them these things. (the deeds). If this is the correct reading, it is obviously a reference not only to the words of the message, but also to all of the signs he did before the people. (See further, 4:1-9 and 30). However, from the concluding language of this Midrash quoted here further — kol had’varim (‘all’ the words) — it is clear that the sense here is also “words,” and reference is to the wording of the message of redemption: pakod pakad’ti. They would have said that his father handed him over these words, as Joseph had handed them over to Levi, and Levi to Kohath, and Kohath to Amram. This was why he was torn up from his father’s house, and when he came and told Israel all the words, they therefore believed him, as it is said, And the people believed.’” The intent of the Rabbis’ words that Joseph handed it over to Levi [and as to why Jacob did not reveal it directly to Levi] is that Jacob revealed the secret to Joseph because of his love for him. With this very same language Joseph made all his brothers swear, and he revealed it to Levi., that with these words the true messenger will announce the approaching redemption. He told them that he gave this [as a secret to Levi] on account of the tradition he received from his father [not to reveal it], and he commanded that the matter remain a secret., the elders of Israel who now knew of the tradition confirmed him as the true redeemer, for they knew that he could not have heard these words from his father as he was torn up from his father’s home when only twelve years old.

אלוקי העברים, seeing that we originated on the far side of the river Euphrates, we have to worship the gods who have their kingdom in that region.

ושמעו לקולך, in all that you will command them.

אלוקי העבריים, the G’d of those who follow the teachings of Ever. (grandson of Shem)

נקרה עלינו, at a time when we had not expected any divine prophecy, and had not asked anything of Him. He commanded us to bring offerings to satisfy His own will, not ours. [the author justifies the use of the root קרה which describes things which happen without one knowing the cause for these happenings. Ed.]

ושמעו לקולך, “they will listen (obey) to your voice.” According to Rashi, the Jews had already been given the code word with which their eventual redeemer would identify himself when both Yaakov and Joseph used the root word פקד in connection with the time of their redemption from slavery in Egypt. I explained this on Genesis 50,24 and 25. Some people raise the question that actually we have no record of Yaakov having said: פקוד יפקוד. According to my explanation on that verse the question has been answered satisfactorily.