Musnad Abu Ya'la al-Mawsili 140:43

Muhammad ibn Bakkar narrated to us, Abbad ibn Abbad al-Muhallabi narrated to us, from Yazid al-Raqashi, from Anas ibn Malik, who said: They said, "O Messenger of Allah, what do you think about one of us who speaks to himself about something—that he would rather fall from the sky and be cut to pieces than speak of it?" The Messenger of Allah, may Allah bless him and grant him peace, said, "That is pure faith."