It is narrated from Imam al-Zuhri (may Allah have mercy on him) that some men of the Ansar, among them Sayyiduna Abu Ayyub (may Allah be pleased with him) and Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), used to give the fatwa that "water is from water," and that there is no ghusl upon a man who goes to his wife and does not ejaculate. When this was mentioned to Sayyiduna Umar, Ibn Umar (may Allah be pleased with them both) and Sayyidah Aishah (may Allah be pleased with her), they rejected it and said: "When the private part passes beyond the private part, ghusl becomes obligatory."
Sayyiduna Sahl bin Sa'd (may Allah be pleased with him), who had lived in the time of the Prophet (peace and blessings be upon him) and was fifteen years old then, said: Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) told me 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 he (peace and blessings be upon him) commanded ghusl.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 774