´It was narrated from Hafsah that the Prophet said:` "Whoever does not intend to fast before dawn comes, should not fast." (Daif)
Related hadith on this topic
Explanation & Benefits
Maulana Ataullah Sajid
Benefits and Issues:
➊ The mentioned narration has been declared weak in its chain by our esteemed researcher, and he has further written that regarding this issue, there is also a narration from Hafsah radi Allahu anha in Sunan an-Nasa’i, which is authentically established in a mawquf (stopped at the Companion) form. See: (Irwa’ al-Ghalil: 30, 25/4, no. 914).
Therefore, the meaning of intending from the night does not mean from the evening, but rather it means that the intention should be made before true dawn (subh sadiq); whenever during the night the intention is formed that one will fast in the morning, it is valid.
➋ This ruling is for obligatory and mandatory (fard and wajib) fasts. The intention for voluntary (nafl) fasts can be made during the day as well. Similarly, if a voluntary fast is being observed, it can be broken at any time during the day without any sin, as will be mentioned in the next hadith.
➌ Some have said that what is meant here is the fast of making up (qada’), vow (nadhar), and expiation (kaffarah), etc.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 1700