Answered by Mufti Mohammed Tosir Miah
Question:
Visiting the dargah in Sylhet and performing 2 rakah nafl is it allowed? According to my knowledge it is forbidden to visit shrines of pious people. jazak allah
Answer:
Allahmah Ibn Abeedeen Shami 
uses the hadith of Musanaf Ibn Abi Shaibah ,
“That the Prophet of Allah 
used to visit the graves of the Martyrs of Uhud every year” to prove one is allowed to visit the graves of the pious predecessors.
Even though some immoral and cultural practices take place at such graves, which are against the teachings of Islam, this should not prevent one from visiting the graves of the pious predecessors. (Raddul Muhtar p.150 v.3 & Dars Tirmizi p.115 v.2)
For clarification on those practices and innovations, which take place at graveyards, please refer to the answer, Can Women visit the graveyard in the Death and Burial section of our website: www.daruliftabirmingham.co.uk
Regarding your question, if by your question you mean to pray two rakaats optional prayer in front of the grave for the deceased then that without any doubt is unlawful and haram in Islam.
The Prophet of Allah said: Let there be curse of Allah upon the Jews and Christians for they have taken the graves of their apostles as places of worship.” (Sahih Muslim)
Saaiduna Abu Hurairah 
states that the Prophet of Allah said, “If I were to make anyone to prostrate for any creation, I would order a woman to prostrate to her husband.” (Sunan Tirmizi)
Mulla Ali Qari writes under this hadith, “Prostration for Ghair-Allah (anyone besides Allah) is not permitted.” (Mirqat p.467 v.3)
Saaiduna Qais bin Saad Radiallahu Anhu states, “I visited the city of Heera where I saw that the people there prostrated for their leaders. I thought in my heart that the Prophet of Allah Sallallahu Alahi Wasalam is much more worthy of prostration.” The Companion then came to the Prophet of Allah Sallallahu Alahi Wasalam related what he witnessed and said that he was more worthy of prostration. The Prophet of Allah Sallallahu Alahi Wasalam then asked him, “If you pass by my tomb, will you prostrate to it?” He answered, “No.” The Prophet of Allah Sallallahu Alahi Wasalam then said, “Don’t ever prostrate to it. If I were to commandprostartion, I would definitely order women to prostrate to their husbands.” (Sunan Abu Dawud)
However, if you are praying two rakaats in the Masjid near to the tomb with the intention of Tahiyatul Masjid (two rakaats when entering the masjid) then that will be permissible.
Saaiduan Abu Qatadah narrates that the Prophet of Allah has said; “When one of you enters the masjid, then let him not sit until he prays two rakaats.” (Sahih Bukhari p.63 v.1)
Saaiduna Abu Dhar has said: “I entered the mosque, and there was the Prophet of Allah sitting alone, so he said: ‘Oh Abu Dhar, Truly the mosque has a greeting, and indeed its greeting is two rakats, so stand up and pray them.’ (Sahih Ibn Hiban)
In the Hanafi Fiqh, if a person was to sit down and then stand up and read his tahiyatul masjid, he will still get the same reward and it will not decrease by sitting down. (Raddul Muhtar p.460 v.2)
Some people may object and say that having a Masjid near the tomb itself is impermissible. The reply to that will be of the story of the Ashab Kahf (People of the Caves) where a Masjid was built near their tombs. (Maariful Qur’an p.601 v.5)
Allah 
in the Holy Qur’an has said:
“…Said those who prevailed in the matter, “We will certainly make a Masjid over them.” (Surah Al-Kahf v.21)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham