Musnad Ahmad 27:1090

Muhammad narrated to us, Shu'bah narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar, that 'Umar had vowed to perform I'tikaf for a day during the Jahiliyyah. He asked the Messenger of Allah (peace and blessings of Allah be upon him) about that, and he commanded him to perform the I'tikaf.