Al-Matalib al-'Aliyah 2753 — Love and brotherhood

It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: "Be moderate and balanced in loving your beloved and friend, for it is quite possible that one day he becomes your enemy and someone you dislike; and likewise, do not exceed limits in hating your enemy, but keep balance and gentleness, for it may be that one day he becomes your beloved and close friend."
Imam al-Tirmidhi recorded this hadith in his Sunan (1997), and Allamah al-Albani declared it sahih (authentic).