Sahih Bukhari, Hadith: Q7188, (34) Chapter. The one who is the most contentious of enemies; and that is, the most quarrelsome person of the opponents.

صحيح البخاري

Sahih Bukhari

كتاب الأحكام

The Book of Al-Ahkam (Judgements)

باب الألد الخصم:

(34) Chapter. The one who is the most contentious of enemies; and that is, the most quarrelsome person of the opponents.