"He should turn to Allāh the Exalted in supplicating at their place (of burial) and frequently make tawassul through them unto Allāh the Exalted, because He, the Glorified and Exalted choose them, honored them, and ennobled them. So, just as He caused them to be a source of benefit in this world, so it will be in the hereafter, nay, even more. So, whoever has a need to be fulfilled, then let him go to them and make tawassul through them, for they are the intermediaries between Allāh the Exalted and His creation. It has been established in the Sacred Law, and Allāh the Exalted knows what He has vouchsafed to them of (Divine) care. That is abundant and well known."

Tawassul and Istighātha
Imām Ibn al-Hājj al-`Abdarī