Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) said: Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) told me that the ruling they used to give, that (ghusl is obligatory only) when water (semen) comes out, was a concession which the Messenger of Allah (peace and blessings be upon him) granted in the early period; then afterwards he (peace and blessings be upon him) commanded us to take a ritual bath (ghusl) (in every case).
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 535