Praise be to Allaah.
After consulting with scholars and muftis, we have learned that there is no known basis in Islam for putting a crescent on top of the minaret. Some scholars forbid doing so and consider it to be something that is newly-innovated in the religion. The practice may also contain some element of imitating the kuffaar, especially if it is proven that the crescent is a symbol used by those who worship heavenly bodies. So we should not use this symbol, and the mosques money should not be spent on something that serves no Islamic purpose. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid