Sunan Ibn Majah, Hadith 611 ⓘ
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.'"
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنْ حَجَّاجٍ، عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " إِذَا الْتَقَى الْخِتَانَانِ وَتَوَارَتِ الْحَشَفَةُ فَقَدْ وَجَبَ الْغُسْلُ " .
also referenced under: Book 1, Hadith 654 (In-Book) & Volume 1, Book 1, Hadith 611 (DarusSalam)