Sahih Bukhari, Chapter: Funerals (Al-Janaaiz)

When the Prophet became ill, some of his wives talked about a church which they had seen in Ethiopia and it was called Mariya. Um Salma and Um Habiba had been to Ethiopia, and both of them narrated its (the Church's) beauty and the pictures it contained. The Prophet raised his head and said, "Those are the people who, whenever a pious man dies amongst them, make a place of worship at his grave and then they make those pictures in it. Those are the worst creatures in the Sight of Allah."