It is narrated from Sayyidah Hafsah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: "Whoever does not make the intention to fast before Fajr, there is no fast for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7907
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
Ali bin Umar al-Hafiz (al-Daraqutni) (may Allah have mercy on him) said: Abdullah bin Abi Bakr narrated it as marfu' (raised to the Prophet), and he is among the trustworthy narrators who narrate hadith as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7909
Nafi' (may Allah have mercy on him) says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) used to say: The fast belongs only to the one who made the intention to fast before Fajr.
Yunus narrates from al-Zuhri from Sayyiduna Ibn Umar (may Allah be pleased with them both), and in one narration from Salim (may Allah have mercy on him), that Sayyiduna Abdullah and Sayyidah Hafsah (may Allah be pleased with them both) both said this, and the opposite has also been said, as Imam Malik (may Allah have mercy on him) narrated.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7910
Sayyidah Aishah (may Allah be pleased with her) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Whoever does not make the intention to fast before the break of dawn, there is no fast for him."
Al-Daraqutni said: Abdullah bin Abbad alone narrated it from al-Mufaddal with this chain, and all of them are trustworthy (thiqat).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7912