弘法筆を選ばす (Koubou fude wo erabazu; “Kuukai doesn’t choose his brush.”)
A master produces masterpieces regardless of what they are given to work with. Quality comes from skill, not merely from good tools. If you find yourself unwilling to start a project because you’re afraid you don’t have the best tools for the job, this is the kotowaza for you. Being a good illustrator isn’t a matter of having the right pens or pencils; being a good musician is a matter of dedication, not of having top-notch instruments (just ask David Grohl!). The saying also makes a good response if someone blames shoddy workmanship on bad tools instead of taking responsibility and aiming to improve.
We have a verb this time! The phrasing is, again, very compact: we start with the proper noun 弘法, followed by 筆 (fude, “writing-brush”), the object-marker particle を (wo), and the verb 選ぶ (erabu; “to choose,” “to select”) in its archaic/literary negative ず (zu) form. A more literal rendition might read “Kuukai not-choose brush.”
選ぶ may also be written as 択ぶ without any change in meaning or pronunciation. It also seems common for people to stick the topic-marker particle は (wa) in after the name.
My source gives an “opposite” saying, 下手の道具立て (or 道具調べ) (Heta no dougu-date/dougu-shirabe, “The unskilled fuss over their tools.”) While the focus of this kotowaza certainly stands in direct contrast to Kuukai’s, I feel like the basic message is similar: mastery is in focusing on your performance rather than on the tools.
All that being said… there’s a story that directly contradicts the kotowaza. Supposedly at one point Kuukai sent the emperor Saga a set of four brushes with instructions on brush choice depending on calligraphic style, causing a prince to comment that the (already extant!?) kotowaza was mistaken. The internet goes on to tell me that Kuukai was actually relatively picky about his brushes, the ability of a master to pick “the right tool for the right job” being a natural and indispensable part of mastery.
(“Mata fenshingu no shiai de makechatta! Atarashii burēdo kawanai to.” “Burēdo no sei ja nai deshou. ‘Koubou fude wo erabazu‘ da yo.”)
[“I lost in another fencing tournament! I have to buy a new blade.” “It’s not the blade’s fault. ‘A good craftsman doesn’t blame his tools,’ you know.”]