سنن النسائي

It was narrated from Al-Hakam, from his father, that when the Messenger of Allah (ﷺ) performed Wudu', he would take a handful of water and do this with it. Shu'bah described it: "He would sprinkle his private parts with it." [1] Shaikh Ibn As-Sunni said: "Al-Hakam (one of the narrators) is Ibn Sufyan Ath-Thaqafi. [1] The purpose is to ward off devilish whispers lest the person think any emission has taken place, and thus think that his Wudu' has been invalidated.