As salaamu alaykum wa rahmatuLlaah,
For nafl fasting, are we allowed to make the "niyyah" day time also, before "zawaal"? What is the "asl" behind fixing the time limit to make the intention of fasting as "zawaal"?
Wa alaykum salam wa rahmatuLlahi wa barakatuHu,
For a supererogatory fast [nafl], one must make the intention anytime before the sun passes its zenith; so long as one did not do anything to invalidate the fast. (Mughni al-Muhtaj v. 2, p. 149-50)
This is supported by the hadith where the Prophet Muhammad would ask his wife A’ishah (Rady Allah anha) if there was anything to eat. When she would reply in the negative, he would fast. And when she would reply in the affirmative, he would not fast. (al-Sunan al-Kubra v. 4, p. 275 with a sahih isnad) In some narrations, the word غذاء is found; this word is specific to food eaten during that part of the day, i.e. before zawal. (Mughni al-Muhtaj v. 2, p. 150)
And Allah knows best.
Answered by Yaqub Abdurrahman