Bukhari

Hadith Commentary:
Even more wretched than the Jews and Christians are those Muslims who have adorned the graves of saints and dervishes, turning them into shops, and there they make people prostrate and present petitions, hang requests, and offer votive offerings.
These people perform these acts from outside the grave, while those saints inside the graves send curses upon them, because all these saints were followers of the Messenger of Allah (sallallahu alayhi wa sallam) and walked according to his pleasure. Indeed, these worshippers of graves are, in the sight of Allah, polytheists and accursed, no matter how much they pray or perform Hajj.
Never be among those people who deceive Allah with a prostration and the Prophet (sallallahu alayhi wa sallam) with a salutation.

Hadith Commentary:
(1)
Among us, there is also a category of Muslims who, like the Jews and Christians, have adorned the graves of saints and turned them into shops. There, they make people prostrate, hang petitions, and offer votive offerings (niyaz).
These people are running their business on the grave, while the saint inside the grave is sending curses upon them.
Such people are accursed in the sight of Allah, even if they are pilgrims (hajis) and those who perform prayers.
(2)
Imam Bukhari rahimahullah has established from this hadith that a "khameesah"—that is, a black blanket with designs—can be used, as the Messenger of Allah sallallahu alayhi wa sallam covered himself with it in his final moments.