Musnad Ahmad 788:113

Mu'ammal ibn Isma'il narrated to us, Hammad ibn Salamah narrated to us, Abu 'Imran al-Jawni narrated to us, from Abu Bakr ibn Abi Musa, from his father, who said: I came to the Prophet, may Allah bless him and grant him peace, and with me were a group of my people. He said: "Rejoice and give glad tidings to those behind you that whoever testifies that there is no god but Allah, being truthful in it, shall enter Paradise." We left the presence of the Prophet, may Allah bless him and grant him peace, to give glad tidings to the people. We were met by 'Umar ibn al-Khattab, may Allah be pleased with him, and he returned with us to the Messenger of Allah, may Allah bless him and grant him peace. 'Umar said: "O Messenger of Allah, will the people then rely [solely] upon this?" The Messenger of Allah, may Allah bless him and grant him peace, remained silent.