Answer:
A dream he had concerning the priests came true.
He was told about Daniel's prophecy concerning him.
Explanation:
When Alexander approached Jerusalem for battle, Jaddua, the high priest, went out to meet him. Together with other priests and in ceremonial dress, he begged for mercy on behalf of Jerusalem. Alexander spared the city, remembering a dream in which he saw priests coming out to meet him in peace. He even offered sacrifices to the God of the Hebrews, Jehovah.
When the priests told of Old Testament prophecies concerning him (Daniel 8:5, 21), Alexander showed the Jews even more favor.
