From the Very Origins of Shirk

Imām Ibn Taimiyyah, may Allāh have mercy on him, said:

Keeping vigil at gravesites, touching them [for blessings], kissing them [likewise], making supplications at them [or] toward them, and the like of that are the [very origin and] foundation of shirk¹ and worship of things [other than Allāh]. It’s due to this the Prophet ﷺ said: “O Allāh, don’t make my grave an object that is worshiped.”

¹treating other than Allāh as an equal of Allāh in something that is only Allāh’s (tr.)

Majmūʿ Al-Fatāwá, vol. 27, p. 79.