Abu Dawud

Benefits and Issues:
It is necessary to make the intention (niyyah) for obligatory fasts before dawn (fajr), and it is preferable that the intention for each fast be made separately. However, it should be kept in mind that intention (niyyah) is the resolve and determination of the heart.
There is no evidence from the Prophet sallallahu alayhi wa sallam or from the noble Companions radi Allahu anhum after him of making a verbal intention for these acts of worship. To insist on a verbal intention is an innovation (bid‘ah).

(1) The researcher of the book has declared the mentioned narration as weak in terms of its chain of transmission, whereas the narration with the same meaning: 2338, has been declared authentic in its mawquf (attributed to a Companion) form. From this, it is understood that according to the researcher of the book, this narration is correct in meaning. Furthermore, other scholars have also declared the mentioned hadith as authentic, and according to their research, the preferred opinion is that the mentioned narration is actionable. And Allah knows best. For further details, see (Dhakheerat al-‘Uqba Sharh Sunan al-Nasa’i: 21/247-251, Irwa’ al-Ghalil: 4/25-30, no. 914).

(2) The scholars have interpreted this hadith as referring to obligatory fasts or making up missed obligatory fasts, and to other obligatory fasts, while excluding voluntary (nafl) fasts from it, as is clearly evident from the numerous narrations mentioned above. In this way, reconciliation can be made between all the ahadith. Therefore, if it becomes known during the day that Ramadan has begun, then the fast can be started at that very moment, regardless of whether one has eaten or drunk anything or not.

1:
Apparently, this is general and includes both obligatory (fard) and voluntary (nafl) fasts, but the majority of scholars (jumhur) have considered it specific to obligatory fasts, and this is also the preferred (rajih) opinion.
The evidence for this is the narration of Aisha (radi Allahu anha), in which it is mentioned that the Prophet (sallallahu alayhi wa sallam) would come to me and ask:
"Is there anything to eat?" If I replied, "No," he would say:
"Then I am fasting."

Lexical Explanation 532:
مَنْ لَّم یُبَیِّتِ ... etc. "Yubayyit" is derived from "tabyit," meaning to intend (make niyyah) for the fast at night.
لَم یَفرِضہُ is from the pattern of "daraba yadribu," meaning he did not make it obligatory upon himself, and this is in the sense that he did not make the intention for the fast.

Benefits and Issues 532:
➊ The mentioned narration has been declared weak in chain (sanadan da'if) by our esteemed researcher, whereas other scholars have considered it authentic (sahih). Furthermore, the esteemed researcher has written in his verification of Sunan Ibn Majah regarding this narration that this narration is also reported in Sunan Nasa'i from Hafsah radi Allahu anha, and that narration is authentically established as mawquf. See: Tahqiq wa Takhrij Sunan Ibn Majah, No. 1700. Shaykh al-Albani rahimahullah has discussed this at length in Irwa' al-Ghalil, from which the opinion of the authenticity of the hadith appears closer to correctness. For further details, see: [أرواءالغليل : 25/4 ، 30 ، رقم : 914]
Therefore, it is necessary to make the intention (niyyah) for an obligatory fast before the appearance of true dawn (subh sadiq). That is, the intention can be made any time after sunset until before the appearance of true dawn. 2. The intention for a voluntary (nafl) fast can be made during the day as well.
➌ Intention (niyyah) is necessary and obligatory because fasting is an act, and for any act, intention is necessary. For each day's fast, a separate intention is a condition. However, the specific words for intention that are uttered by the tongue are an innovation (bid'ah), because intention is an act of the heart; the tongue has nothing to do with it, nor is this established from the Prophet sallallahu alayhi wa sallam or the noble Companions radi Allahu anhum ajma'in.

Narrator of the Hadith:
Hafsah bint Umar ibn al-Khattab radi Allahu anha was first married to Khunays ibn Hudhafah Sahmi radi Allahu anhu. She migrated with him. After the Battle of Badr, he passed away, so the Messenger of Allah sallallahu alayhi wa sallam married her and admitted her into his household in the third year of Hijrah. She passed away at the age of sixty in Sha'ban, 45 AH.