Musnad Ahmad 32:1844

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from Sufyan, from 'Abdullah ibn 'Isa, he said: Jabr ibn 'Abdullah narrated to me, from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, that he said: "A mudd [a measure] is sufficient for one of you for ablution."