Where can I found the placeholder schema for camel?

Question

I want to use the following syntax to define a property attribute in a camel context like this:

<aggregate prop:completionSize="my.property" completionTimeout="1500" strategyRef=MyAggregationStrategy">
                ....
</aggregate>

and I cannot find anymore the schema xmlns:prop="http://camel.apache.org/schema/placeholder

How can I solve this problem?