Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not precede Ramadan by one or two days, unless it is a person who has already been fasting; then he may fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7943