This site says that
Every devata is said to be mantra-baddha, that is, the devata is obliged to fulfill the purpose for which the mantra/devata is invoked.
Previously also I have come across this belief that devatas invoked in yajnas are bound to fulfill purpose of yajna if properly performed because they are bound by mantras. Which verses from Vedas(Samhita, Brahmana, Aranyaka, Upanishadas) support this belief that devatas are bound by mantras?