Musnad Ahmad 758:1

Waki' narrated to us, Zam'ah narrated to us, from 'Isa ibn Yazdad, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you urinates, let him flick his private part three times." Zam'ah said once: "For that is sufficient for him."