וַיֹּאמְר֗וּ שָׁל֛וֹם לְעַבְדְּךָ֥ לְאָבִ֖ינוּ עוֹדֶ֣נּוּ חָ֑י וַֽיִּקְּד֖וּ וישתחו [וַיִּֽשְׁתַּחֲוּֽוּ׃]
And they said: ‘Thy servant our father is well, he is yet alive.’ And they bowed the head, and made obeisance.
Rashi on Genesis
ויקדו וישתחו AND THEY BOWED THE HEAD AND PROSTRATED THEMSELVES — in recognition of his enquiry regarding their welfare. The root קדד denotes bowing the head: the verb שחה in the Hiphil denotes prostration upon the ground (Megillah 22b).
Sforno on Genesis
שלום לעבדך לאבינו עודנו חי, also your servant our father is well. He does not enjoy the serenity of the dead but he is still very much alive.
Radak on Genesis
ויקדו וישתחוו, when they prostrated themselves this was with the intention of making an obeisance to their father after whom Joseph had inquired (although outwardly, this was not noticeable).