Sunan an-Nasa'i, Book 26, Hadith 136 (In-Book) ⓘ

It was narrated that Al-Bara' said: "I met my maternal uncle who was carrying a flag (for an expedition) and I said: 'Where are you going?' He said: 'The Messenger of Allah is sending me to a man who has married his father's wife after he died, to strike his neck or kill him.'"

أَخْبَرَنَا أَحْمَدُ بْنُ عُثْمَانَ بْنِ حَكِيمٍ، قَالَ حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا الْحَسَنُ بْنُ صَالِحٍ، عَنِ السُّدِّيِّ، عَنْ عَدِيِّ بْنِ ثَابِتٍ، عَنِ الْبَرَاءِ، قَالَ لَقِيتُ خَالِي وَمَعَهُ الرَّايَةُ فَقُلْتُ أَيْنَ تُرِيدُ قَالَ أَرْسَلَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم إِلَى رَجُلٍ تَزَوَّجَ امْرَأَةَ أَبِيهِ مِنْ بَعْدِهِ أَنْ أَضْرِبَ عُنُقَهُ أَوْ أَقْتُلَهُ ‏.‏