Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.020B.

Section : One should sprinkle his penis and underwear with water after urination.

Make sure that he has cleansed himself. If one finds some dampness in his clothes after so doing, he can content himself by saying, “That is just water.” This is based on the hadith related by al-Hakim ibn Sufyan or Sufyan ibn al-Hakim who said, “When the Messenger of Allah, upon whom be peace, urinated, he would wash and sprinkle (his penis).” In another narration it states, “I saw the Messenger of Allah urinate, after which he sprinkled water over his penis.” Ibn ‘Umar used to sprinkle his penis until his underwear became wet.