As you can see in this link, Banu Hashem is not بنوهاشم, but بنی هاشم in Arabic.
But u is used mostly for و and not for ی.
I'm puzzled here and I can't find a good answer for this.
Could you please help?
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The wikipedia page you link doesn't seem to show the form you see to me. Regardless, I suspect this is an issue of case
بنو هاشم with a wāw is the nominative form of Banū Hāshim, and is used when the noun is the subject of the sentence
بنی هاشم with a yāʼ is the accusative and genitive form, Banī Hāshim, and is used when the noun is the object of a verb, preposition, or is attached to by a noun in the construct state
The macron (the line over the vowel) indicates that it is long