It was narrated from 'Amr bin Shu'aib from his father, that his grandfather said:
"The Messenger of Allah said: 'When the two circumcised parts meet, and (the tip of the penis) disappears, then bath is obligatory.'" (ES)
English reference (ES): Vol. 1, Book 1, Hadith 611 (ES)
Arabic reference (ES): Book 1, Hadith 654 (ES)