Sahih Bukhari, Chapter: Funerals (Al-Janaaiz)

Allah's Apostle in his fatal illness said, "Allah cursed the Jews and the Christians, for they built the places of worship at the graves of their prophets." And if that had not been the case, then the Prophet's grave would have been made prominent before the people. So (the Prophet ) was afraid, or the people were afraid that his grave might be taken as a place for worship.