Musnad Ahmad 27:517

Al-Walid (meaning Ibn Muslim) narrated to us, Al-Awza’i narrated to us, Al-Muttalib ibn Abdullah ibn Hantab narrated to me, that Ibn Abbas "used to perform ablution once [for each limb], and he attributed that to the Messenger of Allah, may Allah bless him and grant him peace." And that Ibn Umar "used to perform ablution three times [for each limb], and he attributed that to the Messenger of Allah, may Allah bless him and grant him peace."