Humayd (may Allah have mercy on him) said that Sayyiduna Anas (may Allah be pleased with him) was asked about the prayer of the Prophet (peace and blessings be upon him) and his voluntary fasting. He said: "When he (peace and blessings be upon him) began fasting in a month, we would say that he (peace and blessings be upon him) did not intend to leave off fasting, and when he (peace and blessings be upon him) began leaving off fasting in a month, we would think that he (peace and blessings be upon him) would not fast that month. At whatever part of the night we wanted to see him (peace and blessings be upon him) praying, we saw him, and when we wanted to see him sleeping, we saw that too."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4734