Sforno on Numbers 28:26/ספורנו על במדבר כ״ח:כ״ו

1בשבועותיכם, on account of the weeks of cutting, harvesting. Compare Jeremiah 5,24 שבועות חקת קציר, “the weeks appointed for the harvest.” The reason for the apparently superfluous letter ב at the beginning of this word here is similar to the use by Scripture of the same letter in constructions such as בנפש יכפר in Leviticus 17,11, or בדם עשהאל in Samuel II 3,27, or בלחם נשכרו in Samuel I 2,5. It is a form of emphasis of the word it introduces.