Ibn Ezra on Ki Sisa 32:27

English: AND HE SAID UNTO THEM. Moshe referred to the Lord as the God of Israel because those who worshipped the calf thought that the Lord was to be identified with the calf. 128 The worshippers of the calf referred to it as the god of Israel (v. 4). Hence Moshe emphasized that the Lord, not the calf, was the God of Israel. AND SLAY EVERY MAN HIS BROTHER. Even if it be his brother. 129 If the worshipper happens to be his brother. Many interpret Who said of his father, and of his mother: I have not seen him (Deut. 33:9) as referring to this incident. 130 See Yoma 66:2: “Who said of his father, and of his mother: I have not seen him, his father refers to the father of his mother, who is an Israelite.”