It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "None of you should precede Ramadan with one or two days of fasting, unless it is a fast that a man has already been keeping; then he may fast that day."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7942