From Sahih Muslim (Hadith 1184):
A'isha reported:The Messenger of Allah (ﷺ) said during his illness from which he never recovered: Allah cursed the Jews and the Christians that they took the graves of their prophets as mosques. She ('A'isha) reported: Had it not been so, his (Prophet's) grave would have been in an open place, but it could not be due to the fear that it may not be taken as a mosque
Reference: Muslim 1184