Musnad Ahmad 2:182

Abu Ahmad Muhammad ibn Abdullah narrated to us, Sufyan narrated to us, from Abdullah ibn Dinar, from Ibn Umar, from Umar: That he came to the Prophet, peace and blessings of Allah be upon him, and said: "I am afflicted by janabah (major ritual impurity)." He commanded him to wash his private part and to perform his ablution for prayer.