Multiple heirs fasting on behalf of the deceased on the same day
28-1-2016 | IslamWeb
Question:
Assalaamu alaykum. In a hadith on the authority of Ibn ‘Abbaas, may Allaah be pleased with him, he said that a man came to the Prophet, sallallaahu ‘alayhi wa sallam, and said, “My mother died and she still had to fast one month. Shall I fast on her behalf?” The Prophet, sallallaahu ‘alayhi wa sallam, said:, “If your mother was indebted, would you pay off her debt?” The man replied in the affirmative. The Prophet, sallallaahu ‘alayhi wa sallam, then said, “The debt of Allaah is more deserving of being paid off.” [Al-Bukhari and Muslim] From the above hadith, it is clear that heirs can fast on behalf of the dead person. Is it allowed for a number of heirs (wife, son, daughter, etc) to fast on the same day? We await your valid reply as soon as possible.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and Messenger.
Whoever passes away while still liable for missed fast-days, regardless of whether those were originally obligatory or are due to vow, the preponderant view in this regard is that it is prescribed for his guardians to make up for such missed fast days on his behalf; however, it is merely recommended for them to do so. Please, refer to fatwa 192211.
It is permissible to make up for the fasts on behalf of the deceased with a number of people on the same day on condition that such a fast does not require succession, such as the expiation of thihaar (i.e. the husband saying to his wife, "You are like my mother to me", i.e. "You are unlawful to me exactly like my mother") and the expiation for breaking the oath based on the scholarly view suggesting that it is obligatory to observe such fast-days successively.
Imaam Al-Bukhari 
allocated a chapter in his book Saheeh Al-Bukhari entitled, "The Chapter of whoever died while owing fasts." Al-Hasan said, "If thirty men fast on behalf of the deceased on the same day, it is permissible..."
Al-Haafith Ibn Hajar wrote, "...However, the permissibility of making up for the missed fast-days on behalf of the deceased is confined to the case in which such a fasting does not require succession given that if many persons fast on behalf of the dead person to make up for the missed days of fasting so that each one fasts a day for instance, then the element of succession is missed..." [Fat-h Al-Baari]
Ibn ʻUthaymeen advised:
"It makes no difference whether they all fast on one day or one fasts on one day and then the second fasts on the next day (and so on) until they have completed the thirty fast-days. However, in case of the expiation of thihaar and the like, the fast-days cannot be divided among the heirs because such a fasting requires succession and each of them would not observe fasting for two consecutive months as required. It is not permissible to divide the two months between more than one person; rather it is stipulated that they should be fasted by one person so that it can truly be said that he fasted two consecutive months. One may ask if it is acceptable that one of the heirs could observe fasting for three days in succession and then abstain from fasting and another one observes fasting for another three days and so on. In fact, this is not acceptable because it could not truly be said that any of them fasted two consecutive months as required. Hence, if the deceased was obliged to fast for two consecutive months, then either one of his heirs volunteers to fast them on his behalf, or rather they may feed one poor person for each day."
Allaah knows best.