The Quran

لا يَحِلُّ لَكَ النِّساءُ مِن بَعدُ وَلا أَن تَبَدَّلَ بِهِنَّ مِن أَزواجٍ وَلَو أَعجَبَكَ حُسنُهُنَّ إِلّا ما مَلَكَت يَمينُكَ ۗ وَكانَ اللَّهُ عَلىٰ كُلِّ شَيءٍ رَقيبًا

Muhammad Asad

No [other] women shall henceforth be lawful to thee¹ - nor art thou [allowed] to supplant [any of] them by other wives,² even though their beauty should please thee greatly -: [none shall be lawful to thee] beyond those whom thou [already] hast come to possess.³ And God keeps watch over everything.

Some commentators (e.g., Tabari) assume that this restriction relates to the four categories of women enumerated in verse 50 above: it is, however, much more probable that it is a prohibition barring the Prophet from marrying any woman in addition to those to whom he was already married (Baghawi, Zamakhshari). Some of the earliest, most outstanding authorities on the Qur'an, like Ibn 'Abbas, Mujahid, Ad-Dahhak, Qatadah, Ibn Zayd (all of them cited by Ibn Kathir), or Al-Hasan al-Basri (quoted by Tabari in his commentary on verses 28-29), link this prohibition of further marriages with the choice between the charms of worldly life and the good of the hereafter with which the wives of the Prophet were confronted on the strength of verses 2~29, and their emphatic option for "God and His Apostle" (cf. note 32 above). All those early authorities describe the revelation of verse 52 - and the assurance which it was meant to convey to the wives of the Prophet - as God's reward, in this world, of their faith and fidelity. Since it is inconceivable that the Prophet could have disregarded the categorical injunction, "No [other] women shall henceforth be lawful to thee", the passage in question cannot have been revealed earlier than the year 7 H., that is, the year in which the conquest of Khaybar and the Prophet's marriage with Safiyyah - his last marriage - took place. Consequently, verses 28-29 (with which, as we have seen, verse 52 is closely connected) must have been revealed at that later period, and not, as some commentators think, in the year 5 H. (i.e., at the time of the Prophet's marriage with Zaynab).
I.e., to divorce any of them with a view to taking another wife in her stead (with the prohibitive accent on the 'supplanting" - i.e., divorcing - of any of his wives).
In my opinion, the expression ma malakat yaminuka (lit., "what thy right hand possesses", or "has come to possess") has here the same meaning as in 4:24, namely, "those whom thou hast come to possess through wedlock" (see surah 4, note 26); thus, the above verse is to be understood as limiting the Prophet's marriages to those already contracted.