"He would sprinkle his private parts with it." [1] Shaikh Ibn As-Sunni said: "Al-Hakam (one of the narrators) is Ibn Sufyan Ath-Thaqafi. [1] The purpose is to ward off devilish whispers lest the person think any emission has taken place, and thus think that his Wudu' has been invalidated.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا تَوَضَّأَ أَخَذَ حَفْنَةً مِنْ مَاءٍ فَقَالَ بِهَا هَكَذَا وَوَصَفَ شُعْبَةُ نَضَحَ بِهِ فَرْجَهُ فَذَكَرْتُهُ لإِبْرَاهِيمَ فَأَعْجَبَهُ قَالَ الشَّيْخُ ابْنُ السُّنِّيِّ قَالَ أَبُو عَبْدِ الرَّحْمَنِ الْحَكَمُ هُوَ ابْنُ سُفْيَانَ الثَّقَفِيُّ ؓ  

"I saw the Messenger of Allah ﷺ performing Wudu' and sprinkling his private area (with water)."  

رَأَيْتُ رَسُولَ اللَّهِ ﷺ تَوَضَّأَ وَنَضَحَ فَرْجَهُ قَالَ أَحْمَدُ فَنَضَحَ فَرْجَهُ