Do you ahve his exact words?
A portion of Summa Theologica is below.
The reason that it is an error to apply "all that exists has a cause other than itself" to the first cause is because, obviously, then it would not be the first cause. It would have a cause and on and on, and "in efficient causes it is not possible to go on to infinity." The basic infinite regress, first cause argument.
I think your statement may be a misstatement of: "There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself…"
The first cause cannot be caused by itself or anything outside itself. It can't be caused at all; it can only be UNcaused.
From Summa Theologica:
I answer that, The existence of God can be proved in five ways…Many thanks for your posts..The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
"…But God is uncaused, as shown above, since He is the first efficient cause."I should point out again, that while Aquinas identifies the first cause as God, I'm not accepting that as a requirement in this discussion, call it what you wish. I'm using the generic for "first cause argument" and including Aquinas' work and others on both the argument and objections.