Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5540
Safiyya reported from some of the wives of Allah's Apostle (may peace be upon him) Allah's Apostle (may peace be upon him) having said: He who visits a divner ('Arraf) and asks him about anything, his prayers extending to forty nights will not be accepted.