The father of Ibn Ayyash (may Allah have mercy on him) said: I came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and said: Some people of the neighbourhood and some of our neighbours have fasted today. He said: "On the basis of sighting the crescent moon?" He said: No. He said: "That I should leave a day of Ramadan unfasted (by not fasting, beyond doubt) and then make it up later is dearer to me than that I should fast a day of Sha'ban (with doubt)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9530