The Prophet (ﷺ) appointed Mu'adh (May Allah be pleased with him) as governor of Yemen, and at the time of his departure, he instructed him thus: "First of all, call the people to testify 'La ilaha illallah' (there is no true god except Allah) and that I (Muhammad) am the Messenger of Allah, and if they accept this (declaration of Faith), then tell them that Allah has enjoined upon them five Salat during the day and night; and if they obey you, tell them that Allah has made the payment of Zakat obligatory upon them. It should be collected from their rich and distributed among their poor."

[Al-Bukhari and Muslim].