Albert Barnes Commentary Acts 7:41

"And they made a calf in those days, and brought a sacrifice unto the idol, and rejoiced in the works of their hands." — Acts 7:41 (ASV)

And they made a calf. This was made of the earrings and ornaments which they had brought from Egypt (Exodus 32:2–4). Stephen introduces this to remind them how prone the nation had been to reject God and walk in the ways of sin.