The title "Mother of the Believers" (Ummul Momineen) was first used in the time of the Holy Prophet (pbuh) following the revelation of the sixth verse of Chapter al-Ahzab concerning the Prophet's wives and their relation to the believers. According to the verse, the Prophet's wives are the believers' mothers. This is a divine injunction which is particular to the Prophet (pbuh) and prerogative of him only. This injunction (likening of the Prophet's wives to the believers' mothers) signifies that the Prophet's wives are in some aspects like the believers' own mothers. Thus, some of the reasons behind the revelation of this injunction can be enumerated as under:
First: Respecting the Holy Prophet's wives
Second: Revering and respecting the Prophet (pbuh) himself
Third: Preventing misuse on the part of some malicious people who aimed to reach, after the demise of the Holy Prophet (pbuh), their evil political objectives by marrying his wives.
1. The epithet "Mother of Believers" [Ummul Momineen] came into being in the wake of the revelation of verse No.6 of Chapter al-Ahzab which says "The Prophet [Muhammad] had a greater claim on the believers than they have on their own selves, and his wives are (as) their mothers." God, the Exalted, considers the Prophet's wives as the believers' spiritual mother in the same way as the Prophet (pbuh) was a spiritual father of the Muslim Ummah.
2. This injunction – i.e. the Prophet's wives are the mothers of the nation – is a divine injunction which is particular to and prerogative of the Prophet only. There is another verse in Chapter al-Ahzab which quite clearly refutes the Prophet's wives being equal with other women: "O' ye wives of the Prophet! Ye are not like any other women."
3. The likening of the Prophet's wives to the believers' mothers is a likening in terms of some effects of motherhood. In other words, the Prophet's wives are in some aspects like the believers' own mothers. The mothers have rights upon their children and there are certain reciprocal rules governing between them. However, when it comes to the Prophet's wives, all the rules that are applicable to one's own mother are not applicable to the Prophet's wives. In fact, apart from the necessity of respecting one's own mother and prohibition of marriage with her, there are other rules applicable to a mother and her own son. For example, a mother inherits her own child and her child inherits her. It is permissible for a child to look at his mother's face. He cannot marry with her mother's daughters from another husband and so on and so forth. However, when it comes to Prophet's wives only two rules are applicable to them: 1- The necessity of respecting them; 2- Prohibition of marriage with them.
4. Firstly, this injunction is meant for respecting the Prophet's wives. Since they were the wives of the Messenger of Allah (pbuh) and related to him, Muslims respected them a great deal. Obviously, they were different from other Muslim women as the holy Quran says in verse 32 of Chapter al-Ahzab.
Secondly, it is meant to respect the character of the Holy Prophet (pbuh). As it is deduced from the context in which the verse has been revealed that a number of the opponents wanted to marry the Prophet's wives after his demise so as to insult him or seek revenge on him.
Thirdly, the injunction was to prevent misuse on the part of some malevolent and malicious people who, after the demise of the Holy Prophet (pbuh) aimed to reach their political ends and vicious aims by marrying his wives. According to Martyr Murtaza Mutahhari, the secret behind the prohibition of marriage with the Prophet's wives was to prevent the next husband from misusing that woman's special status and reputation lest they, who were highly respected by people, should be used as instruments in the hands of selfish and adventurous people in political and social matters.
 - Al-Ahzab: 6
 - Tabatabai, Muhammad Hussein, Tafsir Al-Mizan (Farsi Translation), vol.16, pg.414.
 - Al-Ahzab:32 "یا نساء النبی لستن کاحد من النساء..."،
 - See: Tafsir Nemouneh, Vol.17, pg.205-207.
 - Murtaza Mutahhari, Collection of Works, vol.19, pg.431, Also, for further information see Tafsir al-Mizan, vol.16, verse 1-54; chapter al-Ahzab; Tafsir Nemouneh, vol.17, same verses, pg.405 and 406.