Musnad Abu Ya'la al-Mawsili 1:19

Al-Hasan ibn Shabib narrated to us, Hushaym narrated to us, Kawthar ibn Hakim narrated to us, from Nafi’, from Ibn ‘Umar, from Abu Bakr al-Siddiq, who said: I said, "O Messenger of Allah, what is the salvation of this matter which we are in?" He said, "Whoever testifies that there is no god but Allah alone, with no partner, then it is salvation for him."