Musnad Abu Ya'la al-Mawsili 110:67

Zuhayr narrated to us, Ibn 'Uyaynah narrated to us, from 'Amr, from Abu al-Sha'tha', from Ibn 'Abbas, who said: I prayed with the Messenger of Allah (may Allah bless him and grant him peace) in Medina eight [rak'ahs] together, and seven [rak'ahs] together.