Dasha Shloki by Shankara – link
According to legend, Adi Shankara chanted this hymn in response to his disciples's final request for him, which was to explain the essence of the teachings of Vedanta. He is believed to have first sung the hymn when he met his guru, Govindapada. In the hymn, Adi Shankara states that the nature of the self is identical to Brahman, which is not identical to the finite and unreal not-self. He also describes the self as that which is one, auspicious, free of attributes and assimilation, and eternal.