Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said to me: "You fast continually and stand in prayer every night." I said: Yes. He (peace and blessings be upon him) said: "When you do that, your eyes will become sunken and your soul will become exhausted. Whoever fasts all year round, none of his fasting will be accepted. Fast three days every month; that is the fasting of the whole year." He said: I said: I have the strength for more than that. He (peace and blessings be upon him) said: "Then fast like Dawud (peace be upon him): he would fast one day and not fast the next, and when he met the enemy he would not flee."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8473