Plutarch wrote several biographies, Julius Caesar's was one of them. According to him, Caesar was warned several times about his assasination by different omens, like his wife's dream before he went to the Senate, or the seer who told him "he'd be harmed no later than the Ides of March".
Shakespeare put this in his own play on Caesar's life, when he makes a soothsayer tell Julius "Beware of the Ides of March".
