When Allah's Apostle (p.b.u.h) sent Muadh to Yemen, he said (to him), ...

Hadith 2:537

Narrated Ibn Abbas:

When Allah's Apostle (p.b.u.h) sent Muadh to Yemen, he said (to him), "YOU are going to people of a (Divine) Book. First of all invite them to worship Allah (alone) and when they come to know Allah, inform them that Allah has enjoined on them, five prayers in every day and night; and if they start offering these prayers, inform them that Allah has enjoined on them, the Zakat. And it is to be taken from the rich amongst them and given to the poor amongst them; and if they obey you in that, take Zakat from them and avoid (don't take) the best property of the people as Zakat."