In cooperative communications, multiple sources communicate with a single destination in the presence of relay nodes. Network coding in cooperative communications allows the relay nodes to combine the data received from the sources and send the linear combination to the destination. The difference in complexity, reliability and overhead is a critical issue when we compare among the different decoders. In this paper, soft-decision maximum a posterior (MAP) decoder that has an optimal reliability and neglected overhead is proposed. Moreover, the Sphere decoder is used to reduce the complexity of the proposed MAP decoder. The proposed implementation proves that the sphere decoder can be used to reduce the complexity of the MAP decoder to about 3% related to the original complexity of the decoder.