Abu Muslim says: I said to Sayyiduna Abu Dharr (may Allah be pleased with him): At which hour is the night prayer most virtuous? He said: I asked the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "The middle of the night, but those who do it are few."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4664