In this paper, we consider the 1-D wave equation where the spatial domain is a bounded interval. Assuming the initial conditions to be known, we are here interested in identifying an unknown source term, while we take the Neumann derivative of the solution on one of the boundaries as the measurement output. Applying a back-and-forth iterative scheme and constructing well-chosen observers (that will be applied in an offline manner), we retrieve the source term from the measurement output in the minimal observation time.