It is narrated from Sayyiduna Iyas bin Mu'awiyah al-Muzani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Praying at night is a must, even if it is only for the time it takes to milk a she-camel or to milk a goat. And whatever is after the Isha prayer counts as the night."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 783