Sahih al-Bukhari 412

It is related that 'A'isha said, "The Prophet, may Allah bless him and grant him peace, liked to begin with the right in all matters as much as possible; in his purification, combing and putting on shoes." XVI: Is it permitted to dig up the graves of idolaters from the time of Jahiliyya and to use the place as a mosque, taking note of the saying of the Prophet, "Allah cursed the Jews who took the graves of their Prophets as mosques." What is disliked concerning praying where there are graves. 'Umar saw Anas ibn Malik praying where there was a grave and said, "The grave! The grave!" But he did not order him to repeat it.