It is narrated from Sayyiduna Talq bin Ali (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) forbade us to fast one day ahead of Ramadan (as a precaution), until we sight the new moon or the count is completed, and then (forbade) that we stop fasting until we sight it or the count is completed.
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8227