That classical economists such as Adam Smith and David Ricardo formulated and justified versions of the Labour Theory of Value is a foundational matter of historical fact in the history of economic thought, requiring no citation.
The claim refers to a well-documented historical fact regarding the evolution of classical economic theory (e.g., Smith, Ricardo, and later Marx discussing value as grounded in labor). While paper [1] touches on the British classical approach to value theory, the claim itself is a standard historical classification that is trivially true by definition within the history of economics and requires no citation.