Musnad Abu Ya'la al-Mawsili 249:7

Al-Hasan ibn Hammad narrated to us, Abu Yahya al-Kufi narrated to us, from Abu Sa'id al-Shami, from Makhul, from Wathilah ibn al-Asqa', from the Prophet, peace and blessings of Allah be upon him, who said: "Count the verses in the voluntary prayer, and do not count them in the obligatory prayer."