Is an intention (niyyah) made during the day for fasting the following day valid?
General Chapter
Al-Mughni
Book of Fasting
Primary text
An intention made during the daylight hours for the following day's fast is not valid unless that intention is maintained until some part of the preceding night. This ruling is supported by the explicit statement that there is no fast for one who has not made the intention from the night (La siyam liman lam yubayyit as-siyam min al-layl). Furthermore, validity requires that the intention be made at the commencement of the act of worship or close to it.
Supporting text
A narration from Ibn Mansur, via Ahmad, suggests that intending a makeup fast (qada' Ramadan) during the day without having intended it from the night is permissible, unless the intention was subsequently revoked. However, Al-Qadi interprets this, correctly, as meaning the intention was maintained until some part of the night passed.