The Prophet ﷺ said, "Whoever does not form the intention to fast during the night, there is no fast for him."
عن النبي ﷺ قال: من لم يبيت الصيام من الليل فلا صيام له.
Abu Dawud (2454), Ibn Khuzayma (1933), and Bayhaqi (4/202) all narrated it through routes from Abdullah bin Wahb, who said, "Ibn Lahia and Yahya bin Ayub narrated to me," from Abdullah bin Abu Bakr bin Hazm, from Ibn Shihab, from Salim, from his father, from Hafsa, the wife of the Prophet ﷺ, and he related it.This is a sahih chain of transmission. There is well-known discussion about Ibn Lahia, but this narration comes through one of the Abadila who narrated from him, and it is also corroborated. Nasai (2332), Tirmidhi (730), and Bayhaqi narrated it through routes from Yahya bin Ayub, from Abdullah bin Abu Bakr, with a similar chain.However, Tirmidhi found fault with it, saying, "The hadith of Hafsa is a hadith we do not know as elevated except through this route. It has been narrated from Nafi, from Ibn Umar, as his own statement, and that is more sound. Likewise, this hadith has also been narrated from Zuhri as a suspended report, and we do not know of anyone who elevated it except Yahya bin Ayub." End of quote.Yahya bin Ayub is Ghafiqi, about whom there is disagreement, though his hadith is hasan when corroborated. I have seen in the chain that Ibn Lahia corroborated him, and Abdullah bin Wahb narrated from Ibn Lahia, being one of the Abadila who narrated from him before his confusion set in, so his hadith is sahih. How much more so, since Yahya bin Ayub also corroborated him. So Tirmidhi's statement, "We do not know it as elevated except through this route," reflects the limits of his own knowledge, and above every possessor of knowledge there is one more knowing.Abu Dawud said, "Layth and Ishaq bin Hazim also both narrated it from Abdullah bin Abu Bakr, in a similar way." This means that four narrators, namely Ibn Lahia, Yahya bin Ayub, Layth, and Ishaq bin Hazim, agreed on elevating this hadith. It appears that a mistake occurred on the part of Abu Dawud, for Layth's narration is not from Abdullah bin Abu Bakr, but rather from Yahya bin Ayub, as Nasai (2331) and Darimi (1740) both narrated it, both from Saeed bin Shurahbil, who said, "Layth narrated to us," from Yahya bin Ayub, from Abdullah bin Abu Bakr, from Salim bin Abdullah, from Abdullah bin Umar, from Hafsa, and he related it. This is Nasai's narration. So Layth bin Saad corroborates Abdullah bin Wahb.As for Ishaq bin Hazim's narration, Ibn Abu Shayba (3/31-32) narrated it from Khalid bin Mukhlad, from him, who said, "Abdullah bin Abu Bakr narrated to me," from Salim, from Ibn Umar, from Hafsa, and he related it. Ibn Majah (1700) also narrated it from him, but Khalid bin Mukhlad, who is Qatawani, is disputed, though there is no harm in him for corroborating narrations.However, Layth bin Saad and Ishaq bin Hazim omitted Ibn Shihab, and both versions are sahih, for Abdullah bin Abu Bakr met Salim, so sometimes he narrated it this way, and other times that way, and all of it is sahih, because he is among the reliable and precise narrators.There is disagreement regarding what is narrated from Zuhri, for it came from him elevated as mentioned before, and it also came from him as a suspended report, as Abu Dawud said following the previous discussion, "Mamar, Zubaydi, Ibn Uyayna, and Yunus Ayli all narrated it from Zuhri as a suspended report attributed to Hafsa." I say, the narrations of Ibn Uyayna, Yunus, and Mamar were recorded by Nasai (4/197), who added among those who narrated it from Zuhri as a suspended report, "Ubaydullah." But the ruling belongs to the one who adds, for Zuhri narrated extensively, so it is not far-fetched that some narrated from him elevated, while others narrated from him as suspended.We have also found Ibn Jurayj among those who narrated from Zuhri as elevated. Nasai (2334) narrated it through the route of Abdul-Razzaq, from him. And Ibn Jurayj was corroborated.In summary, the hadith of Hafsa is sahih, both elevated and suspended. The suspended version is more sound, and the elevated version is below it in soundness, but it contains an additional piece of information which is accepted by the majority of scholars. This addition is strengthened by the fact that such a ruling could only originate from the Lawgiver, given what depends on it in terms of obligation or its absence, and this is a strong indication supporting the elevation of the hadith, even though its narrators are of a lower standing than those of the suspended version.It has a daif corroborating witness from Ayesha, narrated by Daraqutni (2/171-172) and Bayhaqi (4/203), but it contains Abdullah bin Abbad Basri, then Masri, who is very daif. Despite that, Daraqutni said, "Abdullah bin Abbad narrated it alone, from Mufaddal, with this chain of transmission, and all of them are reliable narrators." Bayhaqi quoted this from him and endorsed it.But this is questionable, since Ibn Hibban said in al-Majruhin (574), in the biography of Abdullah bin Abbad Basri, that he was a shaykh who settled in Egypt and used to invert reports, narrating from Mufaddal bin Fadala, from Yahya bin Ayub, from Yahya bin Saeed, from Amra, from Ayesha, from the Prophet ﷺ, and he related the hadith. He said, "This is inverted, for it is actually with Yahya bin Ayub, from Abdullah bin Abu Bakr, from Zuhri, from Salim, from his father, from Hafsa. In something resembling this, Rawh bin Faraj Abu Zinbaa narrated from him a fabricated collection." End of quote. So this corroborating witness cannot be relied upon.It has another corroborating witness from Maymuna bint Saad, who said, "I heard the Messenger of Allah ﷺ say, 'Whoever resolves to fast from the night, let him fast, and whoever wakes up without having resolved on it should not fast.'" Daraqutni (1/173) narrated it, but it contains Waqidi, who is accused of dishonesty.The jurisprudence of the hadith. The apparent meaning of the hadith of this chapter indicates that whoever does not intend to fast from the night has no fast, and this is the view of Ahmad, Malik, and Shafii, because fasting is an act of worship that requires intention, like prayer and the rest of acts of worship. What is meant by intention is the heart's resolve to fast, along with partaking of the pre-dawn meal, and being conscious that one is avoiding the things that break the fast. There is no need to verbally utter the intention.As for someone who did not know that the month of Ramadan had begun until after the break of Fajr, he must refrain from the things that break the fast for the rest of his day, since it is a day of Ramadan, and he must make it up, since he did not form the intention to fast before dawn. This applies to fasting Ramadan, making up for Ramadan, or fasting a vow. A single intention suffices for the whole month of Ramadan according to Imam Ahmad. As for voluntary fasting, it is permissible for one to form the intention after daybreak, provided he has not consumed anything that breaks the fast, since this has been established as sahih from the Messenger of Allah ﷺ.See for more, Minna Kubra (3/278).