Sahih Ibn Khuzaymah

Sayyiduna Sahl Ansari رضي الله عنه, who lived during the time of the Prophet صلى الله عليه وسلم and was fifteen years old in the blessed era of the Prophet صلى الله عليه وسلم, says that Sayyiduna Ubayy bin Ka'b رضي الله عنه narrated to me: The fatwa that the noble Companions رضي الله عنهم used to give was that "water is due to water"—ghusl becomes obligatory upon the emission of semen. In the beginning of Islam, the Messenger of Allah صلى الله عليه وسلم had granted concession regarding this, but later the Prophet صلى الله عليه وسلم ordered to perform ghusl. Imam Abu Bakr رحمه الله narrates from his teacher Ahmad bin Mani' رحمه الله, with his chain, from Sayyiduna Ubayy bin Ka'b رضي الله عنه that he said: "Water is due to water." This fatwa was a concession in the beginning of Islam, then it was prohibited. The Imam mentions another chain as well.