Midrash Rabbah - Exodus I:29

AND WHEN HE SAW THAT THERE WAS NO MAN (II, 12)-for he saw that the smitten man would no longer live. [footnote states: "So severe were the blows of the Egyptian, that he did not think the Hebrew could live; he thought none witnessed his act."]

Rabbi Judah said: He saw THAT THERE WAS NO MAN who would be zealous for God and slay him. [footnote states: "So he acted on the Rabbinic advice: 'Where there is no man, be thou the man.']