One should not observe more fasts than the fasts of Dawud (alayhis salam), because these are the most virtuous. If someone observes more than this, he will not attain any additional reward. If this happens occasionally in a month, that is a different matter, as the Prophet (sallallahu alayhi wa sallam) would often fast in Sha'ban; however, to do so continuously is prohibited.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2375