Tangible evidence is really important. Get things in writing when you can. If you are in a legal dispute, third parties typically do not want to get involved. This is true even though you can require them to testify with a subpoena. If you think a third party said something to support your case, they may not remember it that way. They may remember it differently, or not at all. This is common.
The general prohibition on hearsay compounds the importance of getting tangible evidence. The simplest definition of hearsay is that it is something someone else said, offered into evidence. There are some exceptions, but you can rarely testify to what someone else said.