Tell People they will get a great reward if they give Allah (you) money
When charity was collected for the poor who collected it? Who made sure it all went to the poor? We don't know but verse 57.18 discusses setting a goodly portion for Allah. Who would be the one that would give Allah the money? Why Muhammad of course.
[57.18] Surely (as for) the charitable men and the charitable women and (those who) set apart for Allah a goodly portion, it shall be doubled for them and they shall have a noble reward.
[58.13] Do you fear that you will not (be able to) give in charity before your consultation? So when you do not do it and Allah has turned to you (mercifully), then keep up prayer and pay the poor-rate and obey Allah and His Apostle; and Allah is Aware of what you do.