Musannaf Ibn Abi Shayba 34:144

Marwan ibn Mu'awiyah narrated to us, from Abu Jabir Muhammad ibn 'Ubayd al-Kindi, who said: Ali said to Ibn al-Kawwa': "Do you know what the first [people] said? Love your beloved moderately, perhaps he will be your enemy one day. And hate your enemy moderately, perhaps he will be your beloved one day."