Rashi on Genesis 17:5/רש"י על בראשית י״ז:ה׳

1כי אב המון גוים FOR A FATHER OF A MULTIDUDE OF NATIONS — An acrostic on his name [אב (ר) הם]. But the ר which was previously in his name — denoting that he was the father (chieftain) only of אב (א) רם] ארם] which was his native country (cf. Genesis 21:7 with verse Genesis 21:10 of the same chapter), whilst now he became father of the entire world — nevertheless was not taken out of his name. For we find also, that the 'י in Sarai’s name continued to complain to God that it had been removed from her name when this was changed to Sarah, until it was placed as an additional letter into the name יהושע, as it is said, (Numbers 13:16) “And Moses called Hoshea (הושע) the son of Nun, Joshua (יהושע)” (Genesis Rabbah 47:1).