It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) performed ablution, he would take a handful of water and sprinkle it on his private parts."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4697
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[حم د ن هـ ك] عن الحكم بن سفيان. صحيح أبي داود ١٥٩.