Parashas Shemos
וַתָּבֹ֕אנָה אֶל־רְעוּאֵ֖ל אֲבִיהֶ֑ן וַיֹּ֕אמֶר מַדּ֛וּעַ מִהַרְתֶּ֥ן בֹּ֖א הַיּֽוֹם׃
When they returned to their father Reuel, he said, “How is it that you have come back so soon today?”
Torah Temimah points to TT on 2:18 (verified).
אל רעואל אביהן — וכי רעואל שמו והלא חובב בן רעואל הוא? אלא מכאן שהתינוקות קורין לאבי אביהן אבא.
English: “To Reuel their father” — is his name Reuel? Is he not Chovav son of Reuel? Rather, from here [we learn] that children call their father’s father “father.”
Custom
Chazal resolve the name discrepancy by noting that grandchildren commonly call a grandfather “father.”