Sayyiduna Iyas bin Mu'awiyah al-Muzani (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Praying (voluntary) prayer at night is a must, even if it is only for the length of time it takes to milk a goat; and whatever (voluntary) prayer is prayed after the Isha prayer counts as night prayer."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Muhammad bin Ishaq, who is a mudallis (one who conceals defects in the chain), and the rest of its narrators are thiqat (trustworthy).