Question: What is the ruling regarding touching the mihraab and the minbar of the al-Masjid an-Nabawee?

Response:
Some of the vistors touch the mihraab and the minbar and the walls of the al-Masjid an-Nabawee and all of this is an innovation.

Shaykh Ibn 'Uthaymeen
al-Bid'u wal-Muhdathaat wa maa laa asla lahu - Page 403

Taken from fatwa-online.com