How many times should one wash one's hands with soap after leaving the toilet, and what is the Shar'i (Islamic legal) evidence for that?

The Prophet, peace and blessings be upon him, was keen to remove any smell or trace of impurity that might cling to his hand after performing istinja (definition)' (cleansing oneself after defecation or urination). He would do this by rubbing his hands with earth or striking them on the ground, as narrated in the hadith (definition) of Abu Hurayrah and Maymunah, may Allah be pleased with them both. This is recommended for someone who performs istinja' with water, to remove any unpleasantness. It is permissible to use something other than earth, such as soap, to achieve the intended purpose.