Musnad Ahmad 27:130

I heard Sufyan, and he said: "He made a vow—meaning—to perform I'tikaf in the Sacred Mosque, so he asked the Prophet, may Allah bless him and grant him peace?" So he commanded him. It was said to Sufyan, "From Ayyub, from Nafi', from Ibn 'Umar, that 'Umar made a vow?" He said: "Yes."