That the Prophet ﷺ used to take him (i.e. Usama) and Al-Hassan (in his lap) and say: "O Allah! Love them, as I love them." (Using translation from Bukhārī 3735)
كَانَ رَسُولُ اللهِ ﷺ يَأْخُذُ بِيَدِي وَيَدِ الْحَسَنِ فَيَقُولُ «اللهُمَّ إِنِّي أُحِبُّهُمَا فَأَحِبَّهُمَا»