XML Stack Overflow

Hy Folks I got the Problem that i have to create an xml in ruby with builder, running on a sinatra server. The Xml is filled with xml tags like this one:

<fu-ba:r test="test1" source="h1">
  somthing
</fu-ba:r>

now i don't know how to get builder to create a tag like this one (the attributes are no Problem). i Tried:

xml.fu-ba:r(......)
xml."fu-ba:r"(.......)
xml. << "fu-ba:r"(......)

Every idea or solution would help a lot, thanks Folks