They (Jews and Christians) took their rabbis and their monks to be their lords besides Allah

It was narrated that ‘Adiy ibn Haatim said: “I came to the Prophet (peace and blessings of Allah be upon him) wearing a gold cross around my neck. He said, ‘O ‘Adiy, cast aside this idol.’ And I heard him reciting from Soorat Baraa’ah [al-Tawbah] (interpretation of the meaning): ‘They (Jews and Christians) took their rabbis and their monks to be their lords besides Allah.’ He said: ‘They do not worship them, but when they permit them something they accept it as permitted, and when they forbid them something they accept it as forbidden.’” (Narrated by al-Tirmidhi and it is a hasan hadeeth).