Shaykh al-Islām—may Allah have mercy on him—said:

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As for the third foundation, it is fasting. Scholars have differed over forming the intention to fast the night before, holding three views.

One group—including Abū Hanīfah—held that any fast, whether obligatory or voluntary, is valid if the intention is formed before solar noon. This is indicated by the hadith concerning ‘Āshūrā’ and the hadith in which the Prophet (may Allah bless him and grant him peace) entered upon ‘Ā’ishah and, finding no food, said, “Then I am fasting.”

In contrast, another group—including Mālik—held that no fast, whether obligatory or voluntary, is valid unless the intention is formed the night before, following the apparent meaning of the hadith of Hafsah