Musnad Ahmad 25:233

Waki’ narrated to us, Zakariya ibn Ishaq al-Makki narrated to us, from Yahya ibn Abdullah ibn Sayfi, from Abu Ma’bad, from Ibn Abbas: That the Messenger of Allah (peace and blessings of Allah be upon him), when he sent Mu’adh ibn Jabal to Yemen, said: "You are coming to a people who are People of the Scripture. So invite them to the testimony that there is no god but Allah and that I am the Messenger of Allah. If they obey you in that, then inform them that Allah, the Almighty and Majestic, has obligated upon them five prayers in every day and night. If they obey that, then inform them that Allah, the Almighty and Majestic, has obligated upon them a charity (Zakat) in their wealth, to be taken from their rich and returned to their poor. If they obey you in that, then beware of the best of their wealth, and fear the supplication of the oppressed, for there is no veil between it and Allah, the Almighty and Majestic."