The Quran

وَإِذ تَقولُ لِلَّذي أَنعَمَ اللَّهُ عَلَيهِ وَأَنعَمتَ عَلَيهِ أَمسِك عَلَيكَ زَوجَكَ وَاتَّقِ اللَّهَ وَتُخفي في نَفسِكَ مَا اللَّهُ مُبديهِ وَتَخشَى النّاسَ وَاللَّهُ أَحَقُّ أَن تَخشاهُ ۖ فَلَمّا قَضىٰ زَيدٌ مِنها وَطَرًا زَوَّجناكَها لِكَي لا يَكونَ عَلَى المُؤمِنينَ حَرَجٌ في أَزواجِ أَدعِيائِهِم إِذا قَضَوا مِنهُنَّ وَطَرًا ۚ وَكانَ أَمرُ اللَّهِ مَفعولًا

(Saheeh International)

And [remember, O Muḥammad], when you said to the one on whom Allāh bestowed favor and you bestowed favor,¹ "Keep your wife and fear Allāh," while you concealed within yourself that which Allāh is to disclose.² And you feared the people,³ while Allāh has more right that you fear Him.⁴ So when Zayd had no longer any need for her, We married her to you in order that there not be upon the believers any discomfort [i.e., guilt] concerning the wives of their claimed [i.e., adopted] sons when they no longer have need of them. And ever is the command [i.e., decree] of Allāh accomplished.

Referring to the Prophet's freed slave, Zayd bin Ḥārithah.
i.e., Allāh's command to the Prophet (ṣ) to marry Zaynab after Zayd divorced her. This was to demonstrate that a man may marry a woman formerly married to his adopted son.
i.e., feared their saying that the Prophet (ṣ) had married the (former) wife of his son (which is prohibited by Allāh in the case of a true, begotten son).
By making known His command.