In the heart of the Muslim Quarter is the oldest mosque in China, originally built in 742 AD. The architecture combines that of typical mosques with Chinese elements.
The mosque is a little difficult to find, but anyone can direct you.
The grounds are very peaceful. We encountered a wedding party wile visiting (bride photo attached).
You are not allowed to enter the mosque unless you are Muslim, but you can look in and take photos.
The Muslim Quarter is in the center of Xi'an. Follow the street behind the Drum Tower (by Starbucks) for one block and you are there. It is a great place to stroll and has lots of food vendors, interesting shops, markets, restaurants, etc.
Well worth the visit if you are in Xi'an!