Muslim has narrated, on the authority of one of the wives of the Prophet (Hafsah - may Allah be pleased with her), that Allah's Prophet said:
"Whoever went to a fortune-teller and asked him about some matter (i.e. of the unseen) and believed him, will have his prayer rejected for forty days."
It is reported on the authority of Abu Hurairah (ra) that the Prophet said:
"Whoever went to a fortune-teller and believed in what he said, has disbelieved in what was revealed to Muhammad."
^Hope that helps! :)