Short answers, pulled from the story.
Sassatavada is a Pali term, also rendered in Sanskrit as śāśvata-dṛṣṭi, usually translated as "eternalism." It refers to the view that certain things, such as the individual self, are permanent and unchanging. The Buddha rejected this view in the nikayas and agamas.
The Buddha rejected Sassatavada on both logical and epistemic grounds. He argued that eternalism, like its opposite annihilationism, overreaches what can be known about the deep structure of reality. He proposed a Middle Way grounded in causality rather than ontology as an alternative.
The opposite of Sassatavada is ucchedavada, or annihilationism, which holds that things simply cease at death or destruction with no continuity. The Buddha rejected both positions equally, placing them as the two extremes his Middle Way navigates between.
Sassatavada holds that the extinction of things means their latency and the production of things means their manifestation. Appearance and disappearance are thus treated as surface phenomena over an unchanging underlying reality. The Buddha considered this account a violation of the principle of the Middle Way.
Sassatavada is discussed in the nikayas, the Pali canonical texts, and in the agamas, the parallel Sanskrit-tradition collection. Both bodies of literature record the Buddha's rejection of eternalism as part of his broader teaching on the Middle Way.
The Middle Way proposed by the Buddha relies on causality rather than ontology. Instead of asking what things fundamentally are, it asks how things arise and cease in dependence on conditions. This framework allows for an account of change and continuity without committing to either an eternal self or the complete destruction of the self.