Question: Are there any innovations which some people fall into (when) at the grave of the Messenger (sal-Allaahu `alayhe wa sallam)?

Response:
From the innovations that take place at the grave of the Messenger (sal-Allaahu `alayhe wa sallam) is much frequentation. For example, whenever he enters the masjid, he goes and says 'salaam' to him and sits near the grave. The Prophet (sal-Allaahu `alayhe wa sallam) said:

((Do not take my grave as a place of (much) frequentation)), [Musnad Ahmad, Musannaf 'Abdur-Razzaaq].

Rather, it is recommneded for one who arrives from a journey to visit it. Also, from the innovations that occur at the Messenger's grave or other than that is the assumption that the supplication there is accepted. On the contrary, it is legislated to send saluations upon him (sal-Allaahu `alayhe wa sallam), and if he wants to supplicate then it is (to be) done in any part of the masjid and if it is done after the prayer then that is best. From that which is done and disliked at the grave of the Prophet (sal-Allaahu `alayhe wa sallam) is raising the voice and requesting from him (sal-Allaahu `alayhe wa sallam) that one's needs be satisfied. This is a major shirk, and that which is obligatory is to be cautious of it.

Shaykh Ibn Fowzaan
al-Bid'u wal-Muhdathaat wa maa laa Asla lahu - Page 240;
Majallah ad-Da'wah - No.1612, Page 37

Taken from fatwa-online.com