Musnad Ahmad 1018:133

Abu al-Nadr narrated to us, al-Mas'udi narrated to us, from Muhajir Abu al-Hasan, from an elder who reached the Prophet, may God bless him and grant him peace, who said: I went out with the Prophet, may God bless him and grant him peace, on a journey. He passed by a man reciting: "Say, 'O disbelievers,'" (Surah al-Kafirun). He said: "As for this one, he has been absolved of polytheism." He said: And another was reciting: "Say, 'He is God, [who is] One,'" (Surah al-Ikhlas). The Prophet, may God bless him and grant him peace, said: "By it, Paradise has become mandatory for him."