天に向かって唾を吐く

From the Buddhist scripture Sutra of Forty-two Chapters:「悪人害賢者、猶仰天而唾、唾不汚天、還汚己身」 ("When evil people harm the wise, it's like spitting at heaven - the spit doesn't soil heaven, but falls back on oneself").

天(てん)に向(む)かって唾(つば)を吐(は)く • (ten ni mukatte tsuba o haku) 

1. An act that backfires, bringing harm to oneself while failing to harm the target, especially when disparaging superiors or sacred things; to shoot oneself in the foot

• 天(てん)に唾(つば)する (tennitsubasuru)
• 天(てん)を仰(あお)いで唾(つば)する (ten'oaoidetsubasuru)

• (abbreviation) 天唾(てんつば) (tentsuba)
• (slang) ブーメラン (būmeran, “boomerang”)