Praise be to Allaah.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him and raise his status among the guided) was asked this question and he answered as follows:
I think that basing da’wah on the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him) comes above all else. This is the view of all of us, beyond a doubt. After that comes that which has been narrated from the Rightly-Guided khaleefahs, the Sahaabah and the imams of Islam among the earlier generations (the salaf).
With regard to what the later and contemporary scholars speak about, it deals with things that have happened more recently, which they know better about. If a person takes from their books that which will benefit him in that respect, then he will have gained a great deal. We know that the contemporary scholars have taken the knowledge that they have from those who came before them, so let us also take from the same source as they did. But there are some matters which emerged later, that they understand better, because these issues did not arise at the time of the salaf in the same manner. Hence I think that one should combine the two good things, relying primarily on the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him) and secondly on the words of the Rightly-Guided khaleefahs and the Sahaabah and the imams of the Muslims, then on what has been written by the contemporary scholars who write about things that did not happen in previous times and were not specifically known to the salaf.