I was terminated by my employer orally in the...

 We would like to inform you that if your inquiry is related to termination of the employment  contract during the probation period, the following information is important:
 The UAE labor law has given the employer the right to terminate the employment contract during the probation period, which should be no more than six months.  If the employer refuses to pay the worker’s salary for the period of his work, the worker may file a complaint at the Ministry of Human Resources and Emiratisation against the employer, as it is the body responsible for receiving labor complaints.
There is no legal provision / articles allowing any company / employer to detain any worker's passport.
Therefore, we recommend you to review the company in an amicable way for the last time in order to reach a settlement and retrieve your passport.
If the agency / the employer refuses to give you back your passport, you can file a complaint at the police station. As well as submitting a complaint at the Ministry of Human Resources and Emiratisation
 Here is the text of the UAE Labor Law related to your inquiry:
Article 37:
A worker may be employed on probation for a period not exceeding six months, during which the employer without notice or severance pay may terminate his services. A worker shall not be placed on probation more than once with the same employer. Where a worker successfully completes the period of probation and continues in employment, the said period shall be calculated as part of his period of service.