It was narrated that Ibn 'Umar said:
"Umar mentioned to the Messenger of Allah (ﷺ) that he became Junub at night, and the Messenger of Allah (ﷺ) said: 'Perform Wudu' and wash you penis, then sleep.'" (ES)
English reference (ES): Vol. 1, Book 1, Hadith 261 (ES)
Arabic reference (ES): Book 1, Hadith 262 (ES)