Answer
(Fatwa: 1386/1353/N=1433)
It is disputed among Ulama to visit grave of a wali or pious person and to request them for dua i.e. requesting him to supplicate Almighty Allah on his behalf for so and so thing. The preferred and correct opinion is that it is unlawful and bid’ah as there is no proof of it in the period from the holy Prophet, Sahaba and their followers. It is also feared that this may lead people gradually to shirk (polytheism). (Baqiyat Fatawa Rashidiya p. 60-61)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband