Musnad Ahmad 30:758

Abu Ahmad narrated to us, Sufyan narrated to us, from Abu al-Jahhaf, from Abu Hazim, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever loves them both has loved me, and whoever hates them both has hated me." Meaning Hasan and Husayn.