"Salafis" indeed claim, on the basis of the Prophet's grave not being initially part of the Prophet's mosque, that its present location inside the mosque is an innovation. This is the position of Nasir al-Din Albani and those who follow him.
The answer to such aberrant and discordant views is in the Prophet's hadith in Sahih Muslim: "The best of my Community are those of the century (qarn) wherein I was sent, then those that came after them." The blessed grave was included in the Masjid in the time of al-Walid ibn `Abd al-Malik on the recommendation of his brother-in-law `Umar ibn `Abd al-`Aziz. Both are Tabi`in and the latter's standing in the Umma then and now is second only to the first four khalifas.
Some of them say: "To visit the Prophet's Mosque is not obligatory, not even after performing Hajj." This, again, is a contrived statement that typifies innnovation, not the discourse of traditional Sunnis. What the overwhelming majority of the books of fiqh say on the question is as follows:
ziyaratu qabr al-nabi sallallahu `alayhi wa sallama mashru`atun bi al-ijma` wa hiya min afdal al-a`mal bi al-ijma`.
"The visit of the Prophet's grave is lawful by consensus,.and it is among the most meritorious deeds by consensus."
Sa`di Abu Habib, Mawsu`at al-ijma` fi al-fiqh al-islami 2:919.
Nawawi, al-Majmu`, al-Idah fi manasik al-hajj and al-Adhkar.
Ibn Jama`a, Hidayat al-salik 3:1384f.
Ibn Hajar, Fath al-Bari 3:51.
Sakhawi, al-Qawl al-badi` p. 160.
Shawkani, Nayl al-awtar 5:97. etc.
Some of them say: "It is not a sin not to visit the Prophet's Mosque. It is recommended to visit it, but not mandatory such as the obligation of performing Hajj." Again these are contrived provisos which have an air of innovation about them, and the false comparison to Hajj is specious. The authors of such statements seem to confuse between visiting the Prophet's mosque, which is recommended generally speaking, and visiting the Prophet himself or the Prophet's grave, which is specifically recommended, as the wording of the authorities all attest. Imam Malik even stated that it should be said: "visit the Prophet" rather than "visit the Prophet's grave."
Some of them say: "One can perform all the rituals of Hajj and not visit the Prophet's Mosque, and yet have the Hajj accepted if Allah wills," and this is clear and manifest innovation as it departs from the practice of the Salaf and Khalaf, for whom the question was whether to begin Hajj by visiting Madina or first going to Mecca. Samhudi states that among the Salaf who considered that one should begin by visiting the Prophet in Madina was `Ilqima, al-Aswad, and `Amr ibn Maymun (Nawawi reports that Imam Ahmad said: "The best of the Tabi`in are Ibn al-Musayyib, then `Ilqima and al-Aswad"), and that Abu Hanifa said in the Fatawa of Abu al-Layth al-Samarqandi): "The best for the pilgrim is to begin with Mecca, and after he has finished his rituals, let him pass by Madina."
As for not going to Madina at all it is unheard of, rather, Ibn `Abd al-Barr and al-Baladhiri relate that Abu Bakrah and Ziyad ibn Abih and others intended pilgrimage on a certain year but postponed it when they realized they would be unable to go to Madina as well! `Abd al-Haqq al-Ishbili (d. 582) stated that the visit is a sunna wajiba or obligatory Sunna, and as for the schools it differs between wajib (some Malikis and some Zahiris), near wajib (Hanafis), and Sunna manduba (Shafi`is and Hanbalis).
Samhudi, Khulasat al-wafa' p. 101.
Nawawi, al-Taqrib wa al-taysir p. 98-99.
Shawkani, Nayl al-awtar 5:94, 97.