It is narrated from Hazrat Aisha (may Allah be pleased with her) that sometimes when the Prophet (peace and blessings be upon him) would go out for the Fajr prayer, drops of water would be dripping from his head, but the obligation of ghusl (ritual bath) upon the Prophet (peace and blessings be upon him) was by choice, not by necessity.
Hadith Referenceمسند احمد / مسند النساء / 26254
Hadith Gradingحکم دارالسلام:إسناده حسن من أجل عاصم ابن بهدلة، خ: 1930، م: 1109