Adam Clarke on 1 Samuel 7:9

Samuel took a sucking lamb - This sucking lamb must have been eight days under its mother before it could be offered, as the law says, Lev 22:27.

Though Samuel was not a priest, yet he offered this sacrifice; or he might have ordered Eleazar to offer it, and still be said to have done it himself: Qui facit per alterum, facit per se; "He who procures a thing to be done, may be said to do it himself." His not sacrificing at the tabernacle was justified by the necessity of the case; neither tabernacle nor ark was at hand.