In most cases the source of an ability is the card itself that has the text that causes the game or players to take some action. Some examples of straight-forard situations like that would be the activated ability of Brass Squire, the triggered ability of Consecrated Sphinx or the static ability of Kemba’s Legion. In all of these examples the source of the abilities are either the card or the permanent represented by the card.

However there are some instances where the source of an ability can be in question, and where it can matter. For example Heavy Arbalest‘s second static ability grants an activated ability to the equipped creature. If anything needs to know what the source of this activated ability is, the answer it will get will be the equipped creature. This can matter as a non-artifact creature that is equipped by a Heavy Arbalest can target a Tel-Jilad Fallen. This is true because the source of this ability is the equipped creature and not the equipment itself.