C. F. Keil and Franz Delitzsch on Numbers 12:13

Num 12:13

Moses, with his mildness, took compassion upon his sister, upon whom this punishment had fallen, and cried to the Lord, “O God, I beseech Thee, heal her.” The connection of the particle נא with אל is certainly unusual, but yet it is analogous to the construction with such exclamations as אוי (Jer 4:31; Jer 45:3) and הנּה (Gen 12:11; Gen 16:2, etc.); since אל in the vocative is to be regarded as equivalent to an exclamation; whereas the alteration into אל, as proposed by J. D. Michaelis and Knobel, does not even give a fitting sense, apart altogether from the fact, that the repetition of נא after the verb, with נא אל before it, would be altogether unexampled.