Soba written with 2 kanji characters, 蕎麦 (meaning soba mugi or buckwheat) as it is today first appeared in "Shugaisho" (Compendium of fragments, attributed to Kinkata TOIN) written in the period of the Northern and Southern Courts (Japan).
なお、蕎麦の2字で「そば」と読むようになった初出は南北朝時代(日本)に書かれた『拾芥抄』である。