Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) says that the fatwa "water is from water" was a concession in the early days of Islam, and this concession was given by the Prophet (peace and blessings be upon him); then after that he (peace and blessings be upon him) commanded ghusl.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 777