It is narrated from Umm al-Mu'mineen Sayyidah Hafsah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever does not make the intention for fasting before Fajr, his fast will not be valid." Imam Darimi (may Allah have mercy on him) said: I say this regarding obligatory and mandatory fasts.