It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Allah Almighty said: "So perform tahajjud with it as an extra (nafilah) for you." [Surah al-Isra: 79] That is, voluntary prayer: standing in prayer at night (qiyam al-layl) was made specific to him (peace and blessings be upon him), and it is obligatory on him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13271