Why did our Prophet Musa (peace be upon him) say, "son of my mother" (ibn umma) and not "son of my mother" (ibn ummi)?

If the vocative is an إضافة (genitive construction) to an إضافة (genitive construction) to the "yā’" of the first person singular (يَاءُ الْمُتَكَلِّم), then the "yā’" is retained, such as: "Yā ibn akhī" (O son of my brother). However, in the cases of "ibna ummī" (son of my mother) or "ibna ‘ammī" (son of my paternal uncle), it is permissible to retain the "yā’" or to omit it while retaining the fatḥah or kasrah to indicate it. The reciters (Qurrā’) have differed concerning (ibna ummī); some recited it with a kasrah on the mīm, and others with a fatḥah, both being valid linguistic forms. The "yā’" of the إضافة was omitted for lightness (تَخْفِيف), because the structure of nida’ (calling/vocative) is based on that, and because these words are frequently used. Harun’s statement to his brother Musa, ﴿يَا ابْنَ أُمَّ﴾ (O son of my mother!), was for entreaty and to emphasize the strong bond, with the vocative particle (yā’) omitted to show agitation.