This existed at least as early as 2nd edition, Shugenja could spend experience points to memorize spells, and the formula is on page 203 of the 2nd edition Players Guide:
Memorizing a spell
... There are two drawbacks to not memorizing a spell. The first is that fishing around for the appropriate spell usually requires an action, which can be deadly in certain situations. The second difficulty comes about when you are not carrying the scroll, and therefore you may not cast the spell.
You can memorize any spell that you are capable of casting by spending 2 Experience Points per spell to be memorized and marking it on your character sheet as an Innate Ability.
In 4th edition, this was changed to the following, explained in the sidebar on page 164, titled How are Spells Cast?:
A skilled shugenja, however, can memorize the incantations by spending a number of Experience Points equal to the Mastery Level of the spell. This allows the spell to be
cast without a scroll.