Muhammad bin Abdullah bin Amr bin Uthman (may Allah have mercy on him) narrates from his mother, Fatimah bint Husayn (may Allah be pleased with her), that a man testified before Ali (may Allah be pleased with him) to sighting the new moon of Ramadan, so he fasted; and I think he said: and he ordered the people to fast. And he said: "Fasting a day of Sha'ban is dearer to me than leaving a fast of Ramadan."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7981