F-1 students may work in the United States, both on and off-campus. On-Campus employment is permitted as long as you are in active F-1 status and maintaining your immigration status. Off-campus employment will always require authorization from either ISSS or USCIS.
Reimbursement Letter Request
Some institutions require a reimbursement letter if you attended a workshop, symposium, etc. and are being reimbursed for your travel/personal expenses.
Note: while you can be reimbursed for expenses, you cannot work without an off-campus employment authorization.