Musnad Abu Ya'la al-Mawsili 1:103

Suwayd ibn Sa'id narrated to us, Suwayd ibn 'Abd al-'Aziz narrated to us, from Thabit ibn 'Ajlan, from Salim ibn 'Amir, who said: I heard Abu Bakr saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Go out and call out to the people: 'Whoever testifies that there is no god but Allah, Paradise is guaranteed for him.'" He said: So I went out, and 'Umar ibn al-Khattab met me and said: "What is the matter with you, O Abu Bakr?" I said: "The Messenger of Allah (may Allah bless him and grant him peace) said to me: 'Go out and call out to the people: Whoever testifies that there is no god but Allah, Paradise is guaranteed for him.'" 'Umar said: "Return to the Messenger of Allah (may Allah bless him and grant him peace), for I fear that they will rely upon it." So I returned to the Messenger of Allah (may Allah bless him and grant him peace), and he said: "What brought you back?" I informed him of 'Umar's statement, and he said: "He spoke the truth."