Plutarch's Lives. Book III #113744

All are agreed that Alexander was descended on his father's side from Herakles through Karanus, and on his mother's from Æakus through Neoptolemus.
We are told that Philip and Olympias first met during their initiation into the sacred mysteries at Samothrace, and that he, while yet a boy, fell in love with the orphan girl, and persuaded her brother Arymbas to consent to their marriage. The bride, before she consorted with her husband, dreamed that she had been struck by a thunderbolt, from which a sheet of flame sprang out in every direction, and then suddenly died away. Philip himself some time after his marriage dreamed that he set a seal upon his wife's body, on which was engraved the figure of a lion. When he consulted the soothsayers as to what this meant, most of them declared the meaning to be, that his wife required more careful watching; but Aristander of Telmessus declared that she must be pregnant, because men do not seal up what is empty, and that she would bear a son of a spirited and lion-like disposition.