What is the ruling on one who abandons the obligatory prayer if congregational prayer is a confirmed Sunnah, and is he sinful for abandoning it?

One who abandons an emphasized Sunnah does not incur sin. However, persistently abandoning it, according to some scholars, is considered a breach of honor (khawārim al-murū'a). As for performing prayer in congregation, it is an obligation, and one who abandons it without a valid legal excuse incurs sin.