He saw the Messenger of Allah perform ablution then take a handful of water and sprinkle it over his private area to remove any doubts about urine drippings.  

أَنَّهُ رَأَى رَسُولَ اللَّهِ ﷺ تَوَضَّأَ ثُمَّ أَخَذَ كَفًّا مِنْ مَاءٍ فَنَضَحَ بِهِ فَرْجَهُ  

"The Messenger of Allah said: 'Jibril taught me (how to perform) the ablution, and he ordered me to sprinkle water underneath my garment, lest a drop of urine leak out after the ablution.'" (Da'if) Other chains with similar wording.  

قَالَ رَسُولُ اللَّهِ ﷺ عَلَّمَنِي جِبْرَائِيلُ الْوُضُوءَ وَأَمَرَنِي أَنْ أَنْضَحَ تَحْتَ ثَوْبِي لِمَا يَخْرُجُ مِنَ الْبَوْلِ بَعْدَ الْوُضُوءِ قَالَ أَبُو الْحَسَنِ بْنُ سَلَمَةَ حَدَّثَنَا أَبُو حَاتِمٍ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ التَّنِّيسِيُّ حَدَّثَنَا ابْنُ لَهِيعَةَ فَذَكَرَ نَحْوَهُ  

"The Messenger of Allah said: 'When you perform ablution, sprinkle water.'"  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا تَوَضَّأْتَ فَانْتَضِحْ  

"The Messenger of Allah performed ablution and sprinkled his private part."  

تَوَضَّأَ رَسُولُ اللَّهِ ﷺ فَنَضَحَ فَرْجَهُ