Sayyidah Hafsah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), says that he (peace and blessings be upon him) said: "Whoever does not make the intention to fast along with Fajr, there is no fast for him."
There has been disagreement from al-Zuhri over this hadith, regarding its chain and whether it is raised to the Prophet (peace and blessings be upon him); Abdullah bin Abi Bakr kept its chain sound and narrated it as raised (marfu'), and he is among the trustworthy and precise narrators.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7908