These two Ayas are the key source:
وَلَا تَنكِحُوا مَا نَكَحَ آبَاؤُكُم مِّنَ النِّسَاءِ إِلَّا مَا قَدْ
سَلَفَ ۚ إِنَّهُ كَانَ فَاحِشَةً وَمَقْتًا وَسَاءَ سَبِيلًا
And do not marry those [women] whom your fathers married, except what
has already occurred. Indeed, it was an immorality and hateful [to
Allah ] and was evil as a way.
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ
وَعَمَّاتُكُمْ وَخَالَاتُكُمْ وَبَنَاتُ الْأَخِ وَبَنَاتُ الْأُخْتِ
وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ
الرَّضَاعَةِ وَأُمَّهَاتُ نِسَائِكُمْ وَرَبَائِبُكُمُ اللَّاتِي فِي
حُجُورِكُم مِّن نِّسَائِكُمُ اللَّاتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ
تَكُونُوا دَخَلْتُم بِهِنَّ فَلَا جُنَاحَ عَلَيْكُمْ وَحَلَائِلُ
أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلَابِكُمْ وَأَن تَجْمَعُوا بَيْنَ
الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ إِنَّ اللَّهَ كَانَ غَفُورًا
Prohibited to you [for marriage] are your mothers, your daughters,
your sisters, your father's sisters, your mother's sisters, your
brother's daughters, your sister's daughters, your [milk] mothers who
nursed you, your sisters through nursing, your wives' mothers, and
your step-daughters under your guardianship [born] of your wives unto
whom you have gone in. But if you have not gone in unto them, there is
no sin upon you. And [also prohibited are] the wives of your sons who
are from your [own] loins, and that you take [in marriage] two sisters
simultaneously, except for what has already occurred. Indeed, Allah is
ever Forgiving and Merciful.
Surat An-Nisā' Aya 22-23.
Allah prohibited 14 classes of women, 7 due to blood relationship, and 7 for others.
Those because of blood relationship are:
- Mothers: every direct female ancestor for you (note that she might on the side of your father too, for example, your father's mother). This includes not only ones mother, but also ones grandmother, great-grandmother, and so on no matter how high her level is.
- Daughters: every direct female descendant of you (again, note that she might be on the side of your son, for example, your son's daughter). This includes not only ones daughters, but also ones granddaughters, great-granddaughters, and so on no matter how high her level is.
- Sisters: every female that shares with you your father or mother (or both of them). This includes half-sisters which share only one parent.
- Aunts: both
- Sisters of every male ancestor for you (even on the side of your mother, such as the sister of your mother's father), and
- Sisters of every female ancestor for you (even on the side of your father, such as the sister of your father's mother).
- Nieces: every female descendant of your brothers and sisters, no matter how low her level is (this count 2 out of 7).
This completes the list of women prohibited because of blood relationship.
Now we come to those prohibited because of milk relationship (breastfeeding), there is a basic hadith here:
The Prophet said about Hamza's daughter, "I am not legally permitted
to marry her, as foster relations are treated like blood relations (in
marital affairs). She is the daughter of my foster brother."
حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا هَمَّامٌ، حَدَّثَنَا
قَتَادَةُ، عَنْ جَابِرِ بْنِ زَيْدٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله
عنهما ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم فِي بِنْتِ حَمْزَةَ
" لاَ تَحِلُّ لِي، يَحْرُمُ مِنَ الرَّضَاعِ مَا يَحْرُمُ مِنَ
النَّسَبِ، هِيَ بِنْتُ أَخِي مِنَ الرَّضَاعَةِ ".
Sahih al-Bukhari 2645 (with many, many other riwayat [versions] ).
And, depending on that hadith, there is a basic rule in Shariaa:
يحرم من الرضاع ما يحرم من النسب
What's prohibited (for marriage) because of milk relationship are the
same as what's prohibited because of blood relationship.
Other translation: Marriage prohibition because of milk relationship
is similar to marriage prohibition because of blood relationship.
Now if a woman breastfeeds a boy, then she is considered (in marital affairs) as his own mother, and blood-relationship prohibition rules are applied with all of her family. And the husband of this woman at the time she breastfed the boy is considered (in marital affairs) as his own father, and (again) blood-relationship prohibition rules are applied with all of his family.
Also milk sisters are prohibited, they're every female breastfed by your (blood) mother or your mother in law (your father's wife). And she is considered (in maritial affairs) as your own sister.
But, the amount of breastfeeding that makes these rules applied is not the same for all scholars, and I'm not going to explain this now. (general speaking two pleasing breadfeeds are enough to make these rules applied).
Now we come to those prohibited because of affinity, they're:
- Wife's mothers: every female ancestor of the wife. And they become prohibited just if the marriage contract is made (even before any intercourse).
- Wife's daughters: every female descendant of man's wife from another husband. At the condition of making an intercourse with this wife, i.e, marriage contract is not enough to prohibit these daughters, an intercourse is needed to make the prohibition applied.
- Daughters in law: the wife of any male descendant, no matter how low her level is.
- Milk Sons' wives: wives of man's milk sons (the rule of milk prohibition applies here).
- Mother in law: the wife of any male ancestor, no matter how high his level is.
Also these three classes have some differences in scholars' views, in affairs of intercourse or just marriage contract... etc. But again, I'm not going to discuss these now.
- Sisters of current wives: a man is not allowed to marry two sisters at the same time, if he marry a woman, then all of her sisters are temporarily prohibited for him. If she die, or they divorce, then her sisters become un-prohibited for him, he can marry any of them (after the divorce period عدة of the wife is over).
And finally, of course he can't marry a married woman! if she divorce, then he can marry her after her divorce period عدة is over.
Sources: Tafseer Al Qurtubi, Ahkam Al-Quran by Ibn Arabi, and Ahkam
Al-Quran by Alkia Alharrasi (may Allah be merciful to them all).
Especially Tafseer Al Qurtubi 5 [105-134].
P.S: this is not a complete list or explanation, for example, I've not mentioned prohibited slave females (because there is no slavery anymore, thank Allah, and because they're very few), and I haven't mentioned some rare cases. If I want to explain and take care of every case and scholar view, this post could run into pages (full of information not interesting for everybody).