The Book of Greetings
وَحَدَّثَنِي أَبُو الطَّاهِرِ، أَخْبَرَنَا ابْنُ وَهْبٍ، قَالَ سَمِعْتُ اللَّيْثَ بْنَ سَعْدٍ، يَقُولُ الْحَمْوُ أَخُ الزَّوْجِ وَمَا أَشْبَهَهُ مِنْ أَقَارِبِ الزَّوْجِ ابْنُ الْعَمِّ وَنَحْوُهُ .Ibn Wahb reported:I heard Laith b. Said as saying: Al-Hamv means the brother of husband or like it from amongst the relatives of the husband, for example, cousin, etc
From Sahih Muslim, whose reports the scholars accept as authentic.