Sayyiduna Ali (may Allah be pleased with him) narrated that when the month of Ramadan came, he would deliver a sermon and then say: "This is the blessed month whose fasting Allah has made obligatory, and He has not made its night prayer obligatory. A man should beware of saying: If so-and-so fasts, I will fast, and when so-and-so breaks the fast, I will break the fast. Beware! Fasting is not refraining from food and drink; rather, it is refraining from lies, falsehood and idle talk. Beware! Do not precede this month. When you see the crescent, fast, and when you see it, break the fast. If it is clouded over you, then complete the count." The narrator said: He used to say this after the Asr prayer or the Fajr prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7955