Ibn Kathir
The `Iddah (Waiting Period) of the Widow
This Ayah contains a command from Allah to the wives whose husbands die, that they should observe a period of `Iddah of four months and ten nights, including the cases where the marriage was consummated or otherwise, according to the consensus (of the scholars).
The proof that this ruling includes the case where the marriage was not consummated is included in the general meaning of the Ayah. In a narration recorded by Imam Ahmad and the compilers of the Sunan, which At-Tirmidhi graded Sahih, Ibn Mas`ud was asked about a man who married a woman, but he died before consummating the marriage. He also did not appoint a Mahr (dowry) for her. They kept asking Ibn Mas`ud about this subject until he said, "I shall give you my own opinion, and if it is correct then it is from Allah, while if it is wrong it is because of my error and because of (the evil efforts of) Satan. In this case, Allah and His Messenger are innocent of my opinion. She has her full Mahr." In another narration, Ibn Mas`ud said, "She has a similar Mahr to that of the women of her status, without stinginess or extravagance." He then continued, "She has to spend the `Iddah and has a right to the inheritance." Ma`qil bin Yasar Ashja`i then stood up and said, "I heard Allah's Messenger ﷺ issue a similar judgment for the benefit of Barwa` bint Washiq." `Abdullah bin Mas`ud became very delighted upon hearing this statement. In another narration, several men from Ashja` (tribe) stood up and said, "We testify that Allah's Messenger ﷺ issued a similar ruling for the benefit of Barwa` bint Washiq."
As for the case of the widow whose husband dies while she is pregnant, her term of `Iddah ends when she gives birth, even if it occurs an instant (after her husband dies). This ruling is taken from Allah's statement:
وَأُوْلَـتُ الاٌّحْمَالِ أَجَلُهُنَّ أَن يَضَعْنَ حَمْلَهُنَّ
(And for those who are pregnant, their `Iddah is until they lay down their burden.) (65:4)
There is also a Hadith from Subay`ah Al-Aslamiyah in the Two Sahihs, through various chains of narration. Her husband, Sa`d bin Khawlah, died while she was pregnant and she gave birth only a few nights after his death. When she finished her Nifas (postnatal period), she beautified herself for those who might seek to engage her (for marriage). Then, Abu Sanabil bin Ba`kak came to her and said, "Why do I see you beautified yourself, do you wish to marry By Allah! You will not marry until the four months and ten nights have passed." Subay`ah said, "When he said that to me, I collected my garments when night fell and went to Allah's Messenger ﷺ and asked him about this matter. He said that my `Iddah had finished when I gave birth and allowed me to get married if I wished."
The Wisdom behind legislating the `Iddah
Sa`id bin Musayyib and Abu Al-`Aliyah stated that the wisdom behind making the `Iddah of the widow four months and ten nights is that the womb might contain a fetus. When the woman waits for this period, it will become evident if she is pregnant. Similarly, there is a Hadith in the Two Sahihs narrated by Ibn Mas`ud stating:
«إنَّ خَلْقَ أَحَدِكُمْ يُجْمَعُ فِي بَطْنِ أُمِّهِ أَرْبَعِينَ يَوْمًا نُطْفَةً، ثُمَّ يَكُونُ عَلَقَةً مِثْلَ ذلِكَ، ثُمَّ يَكُونُ مُضْغَةً مِثْلَ ذلِكَ، ثُمَّ يُبْعَثُ إِلَيْهِ الْمَلَكُ فَيَنْفُخُ فِيهِ الرُّوح»
((The creation of) a human being is put together in the womb of his mother in forty days in the form of a seed, and next he becomes a clot of thick blood for a similar period, and next a morsel of flesh for a similar period. Then, Allah sends an angel who is ordered to breathe life unto the fetus.)
So, these are four months and ten more days to be sure, as some months are less (than thirty days), and the fetus will then start to show signs of life after the soul has been breathed into it. Allah knows best.
The `Iddah of the Slave Mother whose Master dies
We should state here that the `Iddah of the slave mother is the same in the case of death, as the `Iddah of the free woman. Imam Ahmad reported that `Amr bin Al-`As said, "Do not confuse the Sunnah of our Prophet for us. The `Iddah of the mother, who is also a servant, when her master dies, is four months and ten nights."
Mourning is required during the `Iddah of Death
Allah said:
فَإِذَا بَلَغْنَ أَجَلَهُنَّ فَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا فَعَلْنَ فِى أَنفُسِهِنَّ بِالْمَعْرُوفِ وَاللَّهُ بِمَا تَعْمَلُونَ خَبِيرٌ
(...then when they have fulfilled their term, there is no sin on you if they (the wives) dispose of themselves in a (just and) honorable manner (i.e., they can marry). And Allah is well-acquainted with what you do.)
This Ayah indicates that mourning for the dead husband is required until the `Iddah is finished. It is also reported in the Two Sahihs that Umm Habibah and Zaynab bint Jahsh narrated that Allah's Messenger ﷺ said:
«لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ وَالْيَوْم الآخِر أن تُحِدَّ عَلى مَيِتٍ فَوْقَ ثَلَاثٍ، إِلَّا عَلى زَوْجٍ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا»
(It is not lawful for a woman who believes in Allah and the Last Day to mourn for more than three days for any dead person except her husband, for whom she mourns for four months and ten days.)
It is reported in the Two Sahihs that Umm Salamah said that a woman said, "O Messenger of Allah ﷺ! My daughter's husband died and she is complaining about her eye, should we administer kohl in her eye" He said, "No," several times upon repeating this question. He then said:
«إنَّمَا هِيَ أَرْبَعَةُ أَشْهُرٍ وَعَشْرٌ، وَقَدْ كَانَتْ إِحْدَاكُنَّ فِي الْجَاهِلِيَّةِ تَمْكُثُ سَنَة»
(It is four months and ten (nights)! During the Jahiliyyah, one of you would mourn for an entire year.)
Zaiynab the daughter of Umm Salamah said (about the pre-Islamic era of ignorance), "When the woman's husband died, she would go into seclusion and would wear the worst clothes she has. She would refrain from wearing perfume or any adornments until a year passed. She would then come out of seclusion and would be given dung that she would throw. Then an animal would be brought out, a donkey, a sheep, or a bird. Then some blood would be drained from it, usually resulting in its death."
In short, the mourning required from a wife whose husband dies, includes not using beautification aids, such as wearing perfume and the clothes and jewelry that encourage the men to seek marriage from the woman. All widows must observe this period of mourning whether they are young, old, free, servant, Muslim or disbeliever, as the general meaning of the Ayah indicates.
Allah also said:
فَإِذَا بَلَغْنَ أَجَلَهُنَّ
(...then when they have fulfilled their term) meaning, when the `Iddah finishes, according to Ad-Dahhak and Ar-Rabi` bin Anas.
فَلاَ جُنَاحَ عَلَيْكُمْ
(there is no sin on you) Az-Zuhri said, "Meaning her Wali (guardian)."
فِيمَا فَعَلْنَ
(if they (the wives) dispose) meaning, the women whose `Iddah has finished. Al-`Awfi said that Ibn `Abbas said, "If the woman is divorced or if her husband dies and then her `Iddah term ends, there is no sin that she beautifies herself, so that she becomes ready for marriage proposals. This is the way `that is just and honorable'." It was reported that Muqatil bin Hayyan gave the same explanation. Ibn Jurayj related that Mujahid said:
فَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا فَعَلْنَ فِى أَنفُسِهِنَّ بِالْمَعْرُوفِ
(...there is no sin on you if they (the wives) dispose of themselves in a just and honorable manner.) "refers to allowed and pure (honorable) marriage." It was also reported that Al-Hasan, Az-Zuhri and As-Suddi said the same.