No, but it is helpful, and it is not required under California law.
In fact, an accident report should be requested by the injured party if none is being provided following a slip and fall causing injury. Also, the injured party should not to forget to ask for a copy even if one cannot be provided. Today, more and more people have cell phones with camaras, and it is a good idea to take pictures of the hazardous substance or condition for any future evidence. I have even had clients take a photo of the accident report after filling one out when it was told a copy could not be provided to them. Gather any and all witness information as soon as possible, and if you can take a picture of witnesses, that is also a good idea.