Sunan Ibn Majah Book 1, Hadith 195
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.
He was sitting with the Prophet who said: 'No person's prayer is complete until he performs ablution properly as Allah has commanded him, washing his face, his arms up to the elbows, wiping his head and his feet up to the ankles.'"
"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.