Musnad Abu Ya'la al-Mawsili 110:74

Zuhayr narrated to us, Ibn ‘Uyaynah narrated to us, from Abu al-Zubayr, from Sa‘id ibn Jubayr, from Ibn ‘Abbas, who said: I prayed with the Prophet, may Allah bless him and grant him peace, eight [rak‘ahs] and seven [rak‘ahs] together. It was said to him: "Why did he do that?" He said: "He wanted not to put his nation into difficulty."