The books of Tehillim, Mishlei, Iyov are often referred as ספרי אמ"ת. Where is the first source making this reference?
According to Chaim Shimon Neuhausen in his essay "תרגום אף ב"אמ"ת"", Otzar Hachaim, vol. 8, 1932, this acronym was created by the Masoretes, though he doesn't write which of the Masoretes was the first to do this.
According to Seligman Baer and Hermann Strack in their edition of Aharon ben Asher's Dikdukei Hate'amim, the expression "קבלה של אמת" used by ben Asher here in reference to the Ketuvim may be an allusion to the ספרי אמ"ת. As his book was one of the very first Tanachic grammar works, it seems that this hint may be the first written usage of the acronym.