Sunan Ibn Majah
461
Sahih+2

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

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi