Majma al-Zawaid 16797 — Reports on the virtue of "La ilaha illallah"

Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him) narrates: I said: "O Messenger of Allah, give me some counsel." The Prophet (peace and blessings be upon him) said: "When you commit an evil deed, follow it with a good deed; it (the good deed) will wipe it (the evil deed) out." The narrator says: I said: "O Messenger of Allah, is 'La ilaha illallah' (none is worthy of worship except Allah) among the good deeds?" The Prophet (peace and blessings be upon him) said: "It is the most excellent of good deeds."
Ahmad recorded it, and its narrators are thiqat (trustworthy), except that the narrator Shimr bin Atiyyah
recorded it from his teachers from Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him), and he did not name any of his teachers.