Imam al-Shafi'i (may Allah have mercy on him) stated that Malik mentioned it likewise and said: If it is clouded over for you, then complete the count of thirty. It is reported from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) mentioned Ramadan and said: "If it is clouded over for you, then complete the number of thirty."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7926