Is it obligatory to wash the hands after urinating if they have not been soiled by impurity, and is this from the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace?

Whoever urinates must perform istijmar and istinja (definition)' only. He is not required after that to wash any of his limbs as long as impurity has not touched them, whether the limb is distant like the hand or close like the testicles. This is because the purpose of istinja' is purification from impurity, and whatever has not been touched by impurity does not require washing. It is recommended to wash the hand after istinja' with something that removes the odor of impurity.