Sayyiduna Anas bin Malik (may Allah be pleased with him) said: "Regarding the Messenger of Allah (peace and blessings be upon him): if you wanted to see him praying at night, you could see him, and if you wanted to see him sleeping, you could see that too."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4733