Why do you need a law for everything?
A salary slip is useful for employees as it provides details such as pay scale, DA credits, debits, and all other features, giving them a bird's eye view of their salary. Please refer to the Shops and Establishment Act at
http://www.rckrishnen.com/statutory-compliance.pdf.
There is something known as Form T, sub-rule vi of rule 11, where a leave card or wage slip is to be issued.