Sadaqah Damashqi says that a man came to Sayyiduna Ibn Abbas (may Allah be pleased with them both) and asked him about voluntary fasts. He replied that the Messenger of Allah (peace and blessings be upon him) used to say: "The best way of observing voluntary fasts is the way of my brother Dawud (peace be upon him); he used to fast one day and break the fast the next day."