Moses said to God, "Behold, when I come to the Children of Israel and say to them, 'The God of your forefathers has sent me to you, ' and they say to me, 'What is His Name?' (*)-- what shall I say to them?"
HASHEM answeered Moses, "I Shall Be As I Shall Be." And He said, "So shall you say to the Children of Israel, 'I Shall Be has sent me to you,' "
(*) note:
This is in itself a Divine Name. God said, "I shall be with them in this sorrow as I shall be with them in other sorrows." To this Moses replied, "An evil in its own time is enough!" I.e., why should You imply to them that there will be future exiles; Is it not enough that they suffer now in Egypt? Accepting Moses' argument, God instructed him to say, "I Shall Be [with them in this sorrow] has sent me to you" (Rashi from Berachos 9b).