In fact many Pāli texts do not start this way, but even so it is distinctively associated with sūtras. The authors of the tantras also began their texts with this word, and developed a number of esoteric associations.
evaṃ mayā śrutaṃ
Earlier on this blog I did my own version of evaṃ in Siddhaṃ script.
Here is my take on the Wartu script bīja based on Tashi's calligraphy.