The Enchiridion of Epictetus contains a number of comments very close to this. It is possible that there is another source that gets even closer to your quotation, but I wonder if Tolle is thinking of these comments of Epictetus and paraphrasing them, or not remembering them precisely. (And of course, I am relying on a translation, too!) Here are some of the claims from the version of the text at the Internet Classics Archive:
Epictetus instructs us that we remember that our children are merely human, and that while we appreciate them as examples of humanity, we should accept that it is unreasonable to be overly attached to them in particular:
- With regard to whatever objects give you delight, are useful, or are deeply loved, remember to tell yourself of what general nature they are, beginning from the most insignificant things. If, for example, you are fond of a specific ceramic cup, remind yourself that it is only ceramic cups in general of which you are fond. Then, if it breaks, you will not be disturbed. If you kiss your child, or your wife, say that you only kiss things which are human, and thus you will not be disturbed if either of them dies.
He recommends avoiding the attitude of attachment or ownership toward our family members:
- Never say of anything, "I have lost it"; but, "I have returned it." Is your child dead? It is returned. Is your wife dead? She is returned.
Elaborating on the theme of not imagining that we own our children or deserve to have them but here connecting it with immortality, this line is perhaps closest to the one you ask about:
- If you wish your children, and your wife, and your friends to live for ever, you are stupid; for you wish to be in control of things which you cannot, you wish for things that belong to others to be your own.