A number of key issues arise in the implementation of scalable multicast protocols for wireless mobile ad hoc networks (MANETs), namely energy consumption and data delivery over unstable/mobile nodes. To improve scalability of these protocols, clustering has been proposed. Clustering allows reducing the number of mobile nodes participating in multicast routing algorithms, which in turn significantly reduces the routing-related control overhead. In this paper, we propose a clustering algorithm, called RSIDS (Restful Stability based Insomnious Distributed Sensors), which considers both stability and residual energy of neighboring nodes when selecting critical nodes (i.e. cluster heads and gateways). RSIDS uses Passive Clustering (in opposition to active clustering) to form the clustering structure. The critical nodes selection enables the selection of most stable nodes with high residual energy as critical nodes; the goal is to minimize re-clustering (and thus re-branching for multicast protocols) that may generate considerable overhead and packet losses and increase the lifespan of the network. We show, via simulations, that RSIDS outperforms existing clustering schemes, in terms of packet delivery ratio and network lifetime, when used with the MAODV (Multicast Ad hoc On demand Distance Vector) routing protocol.