Ibn 'Abbas (May Allah be pleased with them) reported:
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].

وعن ابن عباس رضي الله عنه، أن النبي ﷺ بعث معاذًا رضي الله عنه إلى اليمن فقال:
"ادعهم إلى شهادة أن لا إله إلا الله وأني رسول الله، فإن هم أطاعوا لذلك فأعلمهم أن الله تعالى افترض عليهم خمس صلوات في كل يوم وليلة، فإن هم أطاعوا لذلك فأعلمهم أن الله افترض عليهم صدقة تؤخذ من أغنيائهم، وترد على فقرائهم" ((متفق عليه)) .