Tosefta zu Schoftim 1:37

Rabbi Yehuda says, [among] all the converts, everyone brings [first fruits] but they do not recite the declaration. [However,] the Kenites, the descendants of the father-in-law of Moses (Judges 1:16), bring [first fruits] and recite the declaration, as it is said (Num. 10:32), "[Moses said to Hovav, son of R'uel the Midyanite, Moses's father-in-law] ... It will be that if you go with us ... [from the goodness that God does to us, we will do good unto you]."