عن إبن عباس رضي الله عنه قال قال رجل من أصحاب النبي ﷺ لو قد مات رسول الله ﷺ لتزوجت عائشة أو أم سلمة فأنزل الله عز وجل ﴿ وما كان لكم أن تؤذوا رسول الله ولا أن تنكحوا أزواجه من بعده أبداً إن ذلكم كان عند الله عظيماً ﴾ قرآن
السنن الكبرى للبيهقي كتاب النكاح باب ما خص به من أن أزواجه أمهات المؤمنين وأنه يحرم نكاحهن من بعده على جميع العالمين
From Ibn ‘Abbas (may Allah be pleased with him) who said: One of the Companions of the Prophet (SAW) said: If the Messenger of Allah (SAW) had died, ‘Aishah or Umm Salama would have gotten married, so Allah Mighty and Sublime sent down { It is not for you to harm the Messenger of Allah, or to ever have marital relations with his wives after him; indeed that would be a grave offense in the sight of Allah } [Al-Ahzab 53].
Al-Bayhaqi, Al-sunan Al-kubra [The Greater Compilation of Sunnah], The book of marital relations, Section: what pertained especially to Him in that His wives were the mothers of the believers, and that after Him, marital relations with them were prohibited to the whole world