This is the well known and famous statement of Shaykh al-Islam Ibn Taymiyyah (Majmoo' al-Fatawaa 4/149):
لا عيب على من أظهر مذهب السلف وانتسب إليه واعتزى إليه، بل يجب قبول ذلك منه بالاتفاق، فإن مذهب السلف لايكون إلا حقاً.
There is no fault (blame) uopn the one who proclaims the madhhab of the Salaf, who ascribes to it and affiliates himself to it. Rather, it is obligatory to accept that from him by unanimous agreement, for the madhhab of the Salaf is never but the truth.
The statement of Ibn Taymiyyah above was a refutation of the Sufi, Ashari known as al-Izz bin Abd al-Salam who made a remark (والآخر يتستر بمذهب السلف), "And another (one) who conceals himself with the madhhab of the Salaf." Ibn Taymiyyah reconnected the Ummah back to the Salaf after a period of four centuries (from 400H to 700H) in which the Ahl al-Kalaam severed the Ummah from the Righteous Salaf, and he aided and supported their way which accounts for the great enmity shown towards him from many different factions of innovators, chief amongst them the Ahl al-Kalaam and the Soofees.
You can also refer to this article from Ibn Taymiyyah who explains that abandoning ascription to the Salaf is a slogan and emblem of the innovators.