Mishkat al-Masabih 3172
Al-Bara’ b. ‘Azib said:
My maternal uncle Abu Burda b. Niyar passed me carrying a standard, and I asked him where he was going. He replied, “The Prophet has sent me to bring him the head of a man who has married his father’s wife.” 
A version by Abu Dawud, Nasa’i, Ibn Majah and Darimi has, “He has ordered me to cut off his head and take his property.” This version has “my paternal uncle” instead of “my maternal uncle.”

Tirmidhi and Abu Dawud transmitted it.
وَعَنِ الْبَرَاءِ بْنِ عَازِبٍ قَالَ: مَرَّ بِي خَالِي أَبُو بردة بن دِينَار وَمَعَهُ لِوَاءٌ فَقُلْتُ: أَيْنَ تَذْهَبُ؟ قَالَ: بَعَثَنِي النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِلَى رَجُلٍ تَزَوَّجَ امْرَأَةَ أَبِيهِ آتِيهِ بِرَأْسِهِ. رَوَاهُ التِّرْمِذِيُّ وَأَبُو دَاوُد