Why do the fuqaha’ regard it as mustahabb to fast on the eleventh (of Muharram) as well as ‘Ashoora’?

Praise
be to Allah.

The
scholars regarded it as mustahabb to fast on the eleventh of Muharram
because it was narrated from the Prophet (blessings and peace of Allah be
upon him) that he instructed people to fast on this day. This is seen in the
report narrated by Ahmad from Ibn ‘Abbaas (may Allah be pleased with him)
who said: The Messenger of Allah (blessings and peace of Allah be upon him)
said: “Fast the day of ‘Ashoora’, and be different from the Jews with regard
to it; fast the day before it or the day after it.” 

The
scholars differed regarding the soundness of this hadeeth. Shaykh Ahmad
Shaakir classed it as hasan, but the editors of the Musnad classed it
as da‘eef. 

It was
also narrated by Ibn Khuzaymah with the same wording. Al-Albaani said: Its
isnaad is da‘eef, because of the poor memory of Ibn Abi Layla, and it was
contradicted by a mawqoof report narrated by ‘Ata’ and others from Ibn
‘Abbaas, the isnaad of which is saheeh according to at-Tahhaawi and
al-Bayhaqi. End quote. 

If the
hadeeth is hasan, that is all well and good. If it is da‘eef, then da‘eef
hadeeths about such matters are tolerated by the scholars, because its
weakness is minor; it is not false or fabricated, and it speaks about
virtuous deeds. Moreover, it is narrated from the Prophet (blessings and
peace of Allah be upon him) that he encouraged fasting in the month of
Muharram. Indeed the Messenger of Allah (blessings and peace of Allah be
upon him) said:

“The best fast after Ramadaan is the month of Allah Muharram.” 
Narrated by Muslim, 1163. 

Al-Bayhaqi narrated this hadeeth in as-Sunan al-Kubra with the
wording mentioned above, and in another report with the wording “Fast one
day before it and one day after it,” saying “and” instead of “or”. 

Al-Haafiz Ibn Hajar narrated it in It-haaf al-Mahrah (2225) as
follows: “Fast one day before it and one day after,” and he said: It was
narrated by Ahmad and al-Bayhaqi with a da‘eef isnaad, because of the
weakness of Muhammad ibn Abi Layla, but he is not the only one who narrated
it; Saalih ibn Abi Saalih ibn Hayy followed him in that. End quote. 

This
report indicates that it is mustahabb to fast on the ninth, the tenth and
the eleventh. 

The
scholars also mentioned another reason for it being mustahabb to fast on the
eleventh, which is to be on the safe side with regard to the tenth day,
because people may be mistaken in sighting the new moon of Muharram, and it
may not be known exactly which day is the tenth. So if the Muslim fasts on
the ninth, the tenth and the eleventh, then he will have fasted ‘Ashoora’
for certain. Ibn Abi Shaybah narrated in al-Musannaf (2/313) from
Tawoos (may Allah have mercy on him) that he used to fast one day before it
and one day after it, for fear of missing it. 

Imam
Ahmad said: whoever wants to fast ‘Ashoora’ should fast on the ninth and the
tenth, unless there is some uncertainty about the months, in which case he
should fast for three days. Ibn Sireen said that. End quote. 

Al-Mughni,
4/441 

Thus
it becomes clear that it is not correct to describe fasting three days as
being an innovation (bid‘ah). 

But if
a person misses fasting on the ninth, and he fasts only on the tenth, there
is nothing wrong with that, and it is not makrooh. But if he adds the
eleventh to it, that is better. 

Al-Mardaawi said in al-Insaaf (3/346): 

It is
not makrooh to single out the tenth for fasting, according to the correct
view. Shaykh Taqiy ad-Deen (Ibn Taymiyah) agreed that it is not makrooh.

End
quote. 

And
Allah knows best.