It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not fast until you see the crescent of Ramadan, and do not break the fast until you see the crescent of Eid. If the sky is cloudy, then complete the number of days of fasting." The narrator says that Sayyiduna Abdullah bin Umar (may Allah be pleased with him) used to fast one day before. The narrator says, I asked, "Is it for welcoming (the month)?" The teacher replied, "Yes."
Hadith Referenceالسنن المأثورة / باب تفسير الفرعة والعتيرة / 404