Rashi, on Isa 35:3

“Strengthen ye the feeble hands, and confirm the weak knees.”

“Strengthen weak hands (חַזְּקוּ) All the prophets who brought tidings of salvation consoled Israel and strengthened their weak hands. (The word) is in the intensive conjugation, and its meaning is (the causative, i.e.,) strengthen others. If he intended to say, “Be strong, you whose hands are weak,” it would be vowelized like (2 Chron. 32:7) “Be strong (חִזְקוּ) and be firm (וְאִמְצוּ).” Now that he says to them to strengthen others and to make others firm, it is vowelized, חַזְּקוּ, אַמְּצוּ. “Be strong (חִזְקוּ), do not fear” is vowelized חִזְקוּ, a form of the ‘kal’ conjugation, meaning ‘you be strong by yourselves,’ as one says to singular (Jos. 1:6), “Be strong and be firm (חֲזַק ֶמָץ),” and he does not say, חַזֵּק.”

Imported from an open dataset — not yet checked against the printed edition.