It is narrated from Sayyiduna Abdullah bin Salam (may Allah be pleased with him), who said: We were travelling with the Messenger of Allah (peace and blessings be upon him) when he (peace and blessings be upon him) heard the people saying: "O Messenger of Allah, which deed is the most excellent?" The Messenger of Allah (peace and blessings be upon him) said: "Faith in Allah and His Messenger, striving (jihad) in the path of Allah, and an accepted Hajj (Hajj mabrur)." Then he (peace and blessings be upon him) heard a voice in the valley saying: "I bear witness that there is no true god except Allah, and that Muhammad is the Messenger of Allah." The Messenger of Allah (peace and blessings be upon him) said: "And I too bear witness, and I too bear witness. No one bears witness to it except that he becomes free of shirk (associating partners with Allah)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13966