Sayyiduna Jabir (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Never abandon the (voluntary) night prayer, even if it is only for as long as it takes to milk a goat."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Baqiyyah bin al-Walid, who has been criticized a great deal.