Statute law is simply the writing into statute (Acts passed by Parliament) the common law that prevailed until it was codified into statute
Common law applied to partnerships in England until 1890. In that year, parliament passed into statute the common law and created the Partnership Act
“The main source”! That’s a tricky one
If by “main” you mean the one with greatest influence / importance then I suppose that it has to be statute law – the Acts of Parliament
But civil law varies from one nation to another so, for example, in an Islamic country, Sharia Law would be the “main” source whereas in France the main source would be the Code Napoleon