Musannaf Ibn Abi Shayba 37:250

Muhammad ibn Ubayd narrated to us, he said: Sadaqah ibn al-Muthanna narrated to me, from his grandfather, Rabah ibn al-Harith, who said: Al-Hasan ibn Ali stood up after the death of Ali and addressed the people. He praised Allah and extolled Him, then said: "Indeed, that which is coming is near, and the command of Allah is occurring, even if the people dislike it. And I, by Allah, do not love that there should be entrusted to me of the affair of the nation of Muhammad, may Allah bless him and grant him peace, what weighs a mustard seed, in which a cupping-glass of blood is shed, ever since I knew what benefits me from what harms me. So, join your kin."