"Now an omer is a tenth part of an ephah" (Exodus 16:36). And the omer: I do not know how much it is. Scripture therefore teaches, saying "the tenth part of an ephah is it." It is one part out of ten in three seah, which are seven quarter-measures and a remainder. And further, this remainder is one part out of five in a quarter-measure.
The Omer Measured as a Tenth of an Ephah
Mekhilta DeRabbi Shimon Ben Yochai 16:33
והעמר איני יודיע כמה ת"ל עשירית האיפה הוא. אחד מעשרה בשלוש סאין שהן שבעת רבעים ועוד: ועוד זה אחד מחמשה ברובע: