Which Sikh scripture was declared the eternal Guru by Guru Gobind Singh in 1708?

The Sikh scripture declared the eternal Guru by Guru Gobind Singh in 1708 was the Guru Granth Sahib.

Before his death, Guru Gobind Singh ended the line of human Sikh Gurus and directed Sikhs to regard the scripture as the eternal Guru. This decision gave the Sikh community a permanent spiritual authority in the form of the Guru Granth Sahib. The declaration is associated with Nanded in 1708, where Guru Gobind Singh spent his final period.

The scripture contains hymns and compositions by Sikh Gurus as well as selected bhagats and other contributors. It is written mainly in Gurmukhi script, though its language includes several regional and literary forms. Sikhs treat the scripture with the respect given to a living Guru: it is installed, read, ceremonially opened, and respectfully carried in gurdwaras. The Guru Granth Sahib is sometimes called the Adi Granth, but Adi Granth more precisely refers to the earlier compilation before the final additions and designation.