Imam Ali (‘a) was leading his army towards the battle field of Nehrwan, when a companion stopped him. He pointed towards the stranger accompanying him, and said, ‘Ya Ameer ul Momineen! This man is an astrologer and wishes to say something to you.’
The astrologer opined, ‘Ya Ameer ul Momineen! Do not proceed at this hour towards the battlefield, because the stars foretell that whoever travels at this hour will meet with failure and great loss. However, if you travel at the auspicious moment you will meet with splendid victory.’
Imam Ali (‘a) looked at him and asked, ‘Can you foretell at what time good will befall you and at what time evil?’ ‘Yes I can.’
‘You lie. Even the Holy Prophet (s) did not claim to know the hidden realities. How can you? A person who has faith in your knowledge does not need God.’
He then addressed his army thus: ‘Beware of the trying to foretell the future or that which is concealed from you. This art is like witchcraft; witchcraft is like idolatry; idol worship will be punished with hell.’ He then raised his head and looked at the heavens above, offered a prayer of total reliance on Allah and, looking at the astrologer, said, ‘I will intentionally defy your prophecy and proceed at once.’
The battle of Nehrwan was won with great ease and success, as compared to the other battles fought in the way of Allah.
Reference : Anecdotes of The Ahlul Bayt (Daastaan e Raastaan ) by Ayatullah Murtadha Mutahhari