It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: The obligation of ghusl (ritual bath) is due to the emission of the water of life (semen).
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11308
Hadith Gradingحکم دارالسلام:حديث صحيح، خ: 180، م: 343 وهذا إسناد حسن