All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The remnants of impurities left after performing Istinjaa' (which is to remove what comes out of the two private parts from the place where it comes out or the place that is nearest it by washing it with water) and good washing are excused, as the Sharia's Maxim states: "Hardship necessities ease." Moreover, the Prophet (PBUH) has used stones for Istinjaa', as mentioned in some authentic narrations.
Yet, stones won't remove the trace and remnants of impurities completely. Therefore, Muslim jurists stated that the trace of impurities at the usual place of discharge, after performing Istinjaa' and good washing, are excused and this won't render ablution invalid Insha`Allah. In conclusion, "The trace and remnants of impurities at the usual place of discharge and one`s clothes, after performing Istinjaa', are excused." [Hashyat E`anat At-Talibeen vol.1/pp.126]. And Allah Knows Best.