Fasting on the day of Ashura, per classical scholars
The classical scholars’ idea of fasting on the day of Ashura is not based on a ‘legal’ fast - fast from Fajr until Maghrib like it is for the Mukhalifeen.
Rather, it consists of abstaining from food and drink, out of sadness for Imam Husayn’s calamity, until ‘Asr time (the afternoon). Not until Maghrib.
Then one consumes turba and after that, may drink food and water.
Thus, “fasting on Ashura” from the lens of the classical scholars cannot be compared to that of the Mukhalifeen, nor seen as true fasting.
This abstaining from food until ‘Asr is based on the fact the Imam & his companions did not eat or drink on the day of Ashura until their deaths which occurred at ‘Asr time.
I present the opinions of Shaykh al-Tusi, Allama Hilli, and Shaykh al-Mufid to highlight this.
(Misbah al-Mutahajjid, page 771)
It is desirable on this day to do ziyara to him (Imam Husayn), and it is recommended to fast these ten days such that if it is the day of Ashura, he refrains from food and drink until after ‘Asr (the afternoon), then he consumes some of the soil (turba).
And on the day of Ashura, the sorrows of the family of Muhammad, peace be upon them, are renewed.
ويستحب في هذا اليوم زيارته، ويستحب صيام هذا العشر فإذا كان يوم عاشوراء أمسك عن الطعام والشراب إلى بعد العصر، ثم يتناولشيئا من التربة، وفي يوم عاشوراء يتجدد فيه أحزان آل محمد عليهم السلام، ويستحب اجتناب الملاذ فيه وإقامة سنن المصايب إلى بعدالعصر على ما قلناه.
(Tahrir al-Ahkam, vol 1, page 507)
“It is desirable to fast the first 10 days, such that when it is the 10th day - he refrains from food and drink until the afternoon, then he eats some of the soil”
ويستحب صوم العشر بأسره، فإذا كان اليوم العاشر أمسك عن الطعام والشراب إلى بعد العصر ثم يتناول شيئا من التربة
Considering many of Shaykh al-Tusi’s positions as seen in Misbah al-Mutahajjid, Tahdheeb al-Ahkam, etc are based on al-Mufid’s.
It is not unlikely that also what Shaykh al-Mufid meant by imsak is refraining from food until ‘Asr.
“And the matter in which one has a choice is the fast on the Day of Ashura - in the form of imsak as remembrance of the of the calamity of Al Muhammad”
(Al Muqn’aa, p 367)
واما الذي صاحبه فيه بالخيار فصوم يوم عاشوراء على وجه الامساك فيه لمصيبة آل محمد
This is because regarding a’mal of the Day of Ashura, al-Mufid mentions the following in Masar al-Shi’a (page 24-25) - a book which consists of al-Mufid’s highlighting his beliefs on matters mentioned in the book:
In his section on ‘amal (deeds) of Day of Ashura, he cites the narration that imsak of food and water is until zawal al-Shams (afternoon / ‘Asr).
And after that one is to eat-non delicious food, similar to what one undergoing a calamity undegoes.
وفي العاشر من المحرم قتل الحسين عليه السلام وجاءت الرواية عن الصادق عليه السلام باجتناب الملاذ فيه وإقامة سنن المصائب، والامساكعن الطعام والشراب إلى أن تزول الشمس، والتغذي بعد ذلك بما يتغذى به أصحاب المصائب كالألبان وما أشبهها دون اللذيذ من الطعاموالشراب.
Thus, al-Mufid’s view is clear that fast of the day of Ashura is not the entire day but until afternoon, just like Shaykh al-Tusi and Allama Hilli.