Citing a source that you found in another source is known as using a secondary source. You must always try to read and cite the original work (the primary source). If it is not possible to do this, you have to cite the original as contained in the secondary source.
Your in-text citation must include both authors: the author(s) of the original source and the author(s) of the secondary source. For example: (Habermehl, 1985, as cited in Kersten, 1987).
In your reference list you should provide the details of the secondary source (the source you read). In this example: the details of Kersten 1987.
Most author-date citation styles do not include the primary source (the source you did not read) in the reference list. However, with the numeric styles details of both the primary and secondary source may be included in the reference list.