Shatapatha Brahmana 8.4.4.6

8.4.4.6

bāhū pañcadaśavatyau | te yatpañcadaśavatyau bhavataḥ pañcadaśau hi bāhū dve bhavato dvau hīmau bāhū pārśvata upadadhāti pārśvato hīmau bāhū

The two containing the Pañkadasa are the arms (or fore-feet). And as to why these are such as contain the Pañkadasa,--the arms are fifteenfold. There are two of them, because these arms are two. He places them on the sides, because these two arms are at the sides.