Reference for Sotah 5:19
ורבי ישמעאל איידי דבעי למיכתב והיא נטמאה והיא לא נטמאה כתיב נמי וקנא את אשתו
— this is voluntary in the opinion of R. Ishmael; but R. Akiba says: It is obligatory. R. Papa said to Abaye — others declare it was R. Mesharsheya who said to Raba: Is this to say that R. Ishmael and R. Akiba differ in this way throughout the Torah, one maintaining that [a precept] is voluntary and the other that it is obligatory? — He replied, They only differ here over texts: And he be jealous of his wife — it is voluntary in the opinion of R. Ishmael; but R. Akiba says: It is obligatory. What is the reason of R. Ishmael? — He holds the same view as that of the following teacher. It has been taught: R. Eliezer b. Jacob says: Since the Torah declares, Thou shalt not hate thy brother in thine heart,<span class="x" onmousemove="('comment',' Ibid. XIX, 17. ');"><sup>17</sup></span>