وَمَا كَانَ لَكُمْ أَن تؤْذُواْ رَسُولَ اللَّهِ وَلاَ أَن تَنكِحُواْ أَزْوَاجَهُ مِن بَعْدِهِ أَبَداً إِنَّ ذَلِكُمْ كَانَ عِندَ اللَّهِ عَظِيماً
(And it is not (right) for you that you should annoy Allah's Messenger, nor that you should ever marry his wives after him (his death). Verily, with Allah that shall be an enormity.)
[(Qur'an Ahzab 33:53)]
Ibn Abi Hatim recorded that Ibn `Abbas said concerning the Ayah;
[وَمَا كَانَ لَكُمْ أَن تؤْذُواْ رَسُولَ اللَّهِ]
(And it is not (right) for you that you should annoy Allah's Messenger,) "This was revealed concerning a man who wanted to marry one of the wives of the Prophet after he died. A man said to Sufyan, `Was it `A'ishah' He said, `That is what they said.'''
This was also stated by Muqatil bin Hayyan and `Abdur-Rahman bin Zayd bin Aslam. He also reported with his chain of narration from As-Suddi that the one who wanted to do this was Talhah bin `Ubaydullah, may Allah be pleased with him, until this Ayah was revealed forbidding that.
Hence the scholars were unanimous in stating that it was forbidden for anyone to marry any of the women who were married to the Messenger of Allah at the time when he died, because they are his wives in this world and in the Hereafter, and they are the Mothers of the believers, as stated previously.
Allah regarded that as a very serious matter, and issued the sternest of warnings against it, as He said:
[إِنَّ ذَلِكُمْ كَانَ عِندَ اللَّهِ عَظِيماً]
(Verily, with Allah that shall be an enormity.) Then He said:
[إِن تُبْدُواْ شَيْئاً أَوْ تُخْفُوهُ فَإِنَّ اللَّهَ كَانَ بِكُلِّ شَىْءٍ عَلِيماً ]
(Whether you reveal anything or conceal it, verily, Allah is Ever All-Knower of everything.) meaning, `whatever you conceal in your innermost thoughts, it is not hidden from Him at all.'
[يَعْلَمُ خَآئِنَةَ الاٌّعْيُنِ وَمَا تُخْفِى الصُّدُورُ ]
(Allah knows the fraud of the eyes, and all that the breasts conceal.) (40:19).
Surah Ahzaab - Tafsir ibn Kathir;