Albert Barnes Commentary 1 Samuel 2:13

"And the custom of the priests with the people was, that, when any man offered sacrifice, the priest`s servant came, while the flesh was boiling, with a flesh-hook of three teeth in his hand;" — 1 Samuel 2:13 (ASV)

The Law of Moses defined exactly what the priest’s portion of every peace offering was to be (Leviticus 7:31–35), and it also gave express directions about burning the fat (Leviticus 7:23–25, 31). It was therefore a gross act of disobedience and lawlessness for Hophni and Phinehas to take more than the Law gave them. This passage provides incidental evidence for the existence of the Levitical law at this time.