Responding to Salam invalidates the prayer, whether done intentionally or unintentionally; because responding to Salam is not part of the remembrances, rather it is speech and conversation, and speech invalidates the prayer whether done intentionally or unintentionally. Jabir, may Allah be pleased with him, said: "We were with the Prophet, peace be upon him, and he sent me on an errand. I returned while he was praying on his mount, and his face was not towards the Qibla, so I greeted him with Salam, but he did not respond to me. When he finished, he said: 'Nothing prevented me from responding to you except that I was praying.'" (Sahih Muslim 1: 384, Sahih Bukhari 1: 407). See: Fath Bab Al-Inayah 1: 302.