Step 1)
wget -nv http://public-repo-1.hortonworks.com/HDP/centos6/2.x/GA/2.2.0.0/hdp.repo -O /etc/yum.repos.d/hdp.repo

Step 2)Find the source package
yum search hadoop |grep source 
yum search hadoop | grep source
hadoop-source.noarch : hadoop-source HDP virtual package
hadoop-yarn-resourcemanager.noarch : hadoop-yarn-resourcemanager HDP virtual
hadoop_2_2_0_0_2041-source.x86_64 : Source code for Hadoop
hadoop_2_2_0_0_2041-yarn-resourcemanager.x86_64 : YARN Resource Manager

Step 3) Install
yum install hadoop_2_2_0_0_2041-source.x86_6

The code ends up at
/usr/hdp/2.2.0.0-2041/hadoop/src/