Ubayy b. Ka’b said : The verdict that water (bath) is necessary when there is emission given by the people (in the early days of Islam) was due to the concession granted by the Messenger of Allah (ﷺ) in the beginning of Islam. He then commanded to take a bath (in such a case). Abu Dawud said : By Abu Ghassan is meant Muhammad b. mutarrif.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ مِهْرَانَ الْبَزَّازُ الرَّازِيُّ، حَدَّثَنَا مُبَشِّرٌ الْحَلَبِيُّ، عَنْ مُحَمَّدٍ أَبِي غَسَّانَ، عَنْ أَبِي حَازِمٍ، عَنْ سَهْلِ بْنِ سَعْدٍ، حَدَّثَنِي أُبَىُّ بْنُ كَعْبٍ، أَنَّ الْفُتْيَا الَّتِي، كَانُوا يُفْتُونَ أَنَّ الْمَاءَ مِنَ الْمَاءِ كَانَتْ رُخْصَةً رَخَّصَهَا رَسُولُ اللَّهِ صلى الله عليه وسلم فِي بَدْءِ الإِسْلاَمِ ثُمَّ أَمَرَ بِالاِغْتِسَالِ بَعْدُ .
sunnah.com