I asked the Messenger of Allah ﷺ about what makes the full bath obligatory, and about the fluid that comes after the fluid. He said, "That is madhi, and every male emits madhi. So wash your private parts and your testicles from it, and perform ablution as you would for prayer."
حدثنا إبراهيم بن موسى أخبرنا عبد الله بن وهب حدثنا معاوية يعني ابن صالح عن العلاء بن الحارث عن حرام بن حكيم
سألت رسول الله ﷺ عما يوجب الغسل وعن الماء يكون بعد الماء فقال ذاك المذي وكل فحل يمذي فتغسل من ذلك فرجك وأنثييك وتوضأ وضوءك للصلاة
[Its isnad is hasan] Tirmidhi transmitted it, in the Book of Purification, Chapter on What Has Been Said Regarding Eating with a Person in a State of Major Impurity or a Menstruating Woman and Their Leftover Water, hadith 133, and Ibn Majah, hadith 651 and 1378, from the hadith of Muawiya bin Salih with this chain, and Tirmidhi said, hasan gharib.