Historical Rules · 1 November 2013
Rule 112.9
Official source text from the 1 November 2013 Rules release
Activated and triggered abilities on the stack aren't spells, and therefore can't be countered by anything that counters only spells. Activated and triggered abilities on the stack can be countered by effects that specifically counter abilities, as well as by the rules (for example, an ability with one or more targets is countered if all its targets become illegal). Static abilities don't use the stack and thus can't be countered at all.