the idolaters are prostrating to it at that very hour; so if the worshippers of the One God pray to Allah, Mighty and Majestic, at that same time, this creates a resemblance to them and may become a stepping-stone that eventually leads to prostration before it. Indeed, two of the greatest factors behind the worship of idols were the making of images and the veneration of graves.

In Sahih Muslim and other collections it is narrated from Abu al-Hayyaj al-Asadi, who said:
“‘Ali ibn Abi Talib said to me, ‘Shall I not send you with the same charge for which the Messenger of Allah, may Allah bless him and grant him peace, sent me? He commanded me not to leave any elevated grave without leveling it, and not to spare any statue without erasing it.’”

And in the two Sahihs it is related that during his final illness he, may Allah bless him and grant him peace, said:
“May Allah curse the Jews and the Christians; they took the graves of their prophets as places of worship,” warning against what they had done.