"Water is for Water." [1] [1] That is, water for peforming Ghusl is obligatory when the ejaculation of water (semen) occurs. (Using translation from Nasāʾī 199)  

أَنَّ النَّبِيَّ ﷺ قَالَ «الْمَاءُ مِنَ الْمَاءِ»  

[Machine] On the authority of Sahl bin Saad Al-Saa'idi, who met and heard from the Prophet ﷺ when he was fifteen years old, at the time of the Prophet's death. He said, "Ubay bin Ka'b informed me that the fatwa they used to issue regarding the saying of Allah 'Water from water' was initially permitted by the Messenger of Allah ﷺ at the beginning of Islam. Then he commanded them to perform ablution after that." Abdullah (bin Mas'ud) said, and others also said, "Al-Zuhri narrated to me from some who were pleased with Sahl bin Saad."  

عَنْ سَهْلِ بْنِ سَعْدٍ السَّاعِدِيِّ وَكَانَ قَدْ أَدْرَكَ النَّبِيَّ ﷺ وَسَمِعَ مِنْهُ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ سَنَةً حِينَ تُوُفِّيَ رَسُولُ اللَّهِ ﷺ قَالَ حَدَّثَنِي أُبَيُّ بْنُ كَعْبٍ أَنَّ الْفُتْيَا الَّتِي كَانُوا يُفْتَوْنَ بِهَا فِي قَوْلِهِ «الْمَاءُ مِنَ الْمَاءِ» رُخْصَةٌ كَانَ رَسُولُ اللَّهِ ﷺ رَخَّصَ فِيهَا فِي أَوَّلِ الْإِسْلَامِ ثُمَّ أَمَرَ بِالِاغْتِسَالِ بَعْدُ قَالَ عَبْدُ اللَّهِ وقالَ غَيْرُهُ قَالَ الزُّهْرِيُّ حَدَّثَنِي بَعْضُ مَنْ أَرْضَى عَنْ سَهْلِ بْنِ سَعْدٍ  

[Machine] "The fatwa that they used to issue, regarding 'water from water', was a permission that the Messenger of Allah ﷺ granted in the early days of Islam or at that time, then he performed ablution afterwards."  

أَنَّ الْفُتْيَا الَّتِي كَانُوا يُفْتَوْنَ بِهَا «الْمَاءُ مِنَ الْمَاءِ» كَانَتْ رُخْصَةً رَخَّصَهَا رَسُولُ اللَّهِ ﷺ فِي أَوَّلِ الْإِسْلَامِ أَوِ الزَّمَانِ ثُمَّ اغْتَسَلَ بَعْدُ