Al-Fadl bin Yasar states: I heard Muhammad bin Ali (perhaps this refers to Imam al-Baqir) when he was asked about this saying of the Prophet (peace and blessings be upon him): "The fornicator is not a believer while he commits fornication; the thief is not a believer while he steals." Muhammad bin Ali drew a large circle on the ground, then drew another circle in its middle, and said: "The first circle is Islam, and the circle in the middle is faith. When a person commits fornication, he goes out of faith into Islam, but nothing except shirk (associating partners with Allah) takes him out of Islam."
Al-Bazzar recorded it; its chain contains a narrator, al-Fadl bin Yasar, whom al-Uqayli declared da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الإيمان / 374