Use left hand for istinja.
Abu Qatadah رضي الله عنه narrates that Rasúlullãh ﷺ said,
“When any one of you urinates, he must not touch his private parts with his right hand. And when he goes to relieve himself, he must not wipe himself with his right hand. And when he drinks, he must not drink in one breath.” [Abu Dawud]
