It was narrated from Hakam bin Sufyan Ath-Thawri that:
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. (DE)
English reference (DE): Vol. 1, Book 1, Hadith 461 (DE)
Arabic reference (DE): Book 1, Hadith 497 (DE)