Sahih Bukhari Volume 2, Book 23, Hadith Number 414.

Narated By ‘Urwa : ‘Aisha said, “The Prophet in his fatal illness said, ‘Allah cursed the Jews and the Christians because they took the graves of their Prophets as places for praying.” ‘Aisha added, “Had it not been for that the grave of the Prophet (p.b.u.h) would have been made prominent but I am afraid it might be taken (as a) place for praying.