Musnad Abu Ya'la al-Mawsili 111:65

Harun ibn Ma'ruf narrated to us, Ibn Wahb narrated to us, Makhramah ibn Bukayr narrated to me, from his father, from Ibn Abbas, who said: Then I mentioned the night prayer, and some of them said that the Messenger of Allah (may Allah bless him and grant him peace) said: "Half of it, a third of it, a quarter of it, [or] the time between two milkings of a she-camel, [or] the time between two milkings of a sheep."