It is narrated from Hazrat Jabir bin Samurah (may Allah be pleased with him) that people used to call Madinah Munawwarah as Yathrib. The Prophet (peace and blessings be upon him) said that Allah has named Madinah as Tayyibah.
Hadith Referenceمسند احمد / أول مسند البصريين / 20899