A man from Banu Sulaym narrates: I was sitting at al-Mirbad when a Bedouin came with the milk of his camels and stopped near us. His camels crowded in on us, so we got up from our place; then his camels crowded in on us a second time. One of those present said: "I think you are mad." He said: "I am not mad. I have a letter from the Prophet (peace and blessings be upon him)." Then he took it out, and it was on a piece of leather. We read it, and in it was written: "Fasting the month of patience and three fasts of every month drive away the rancour (or evil whisperings) of the chest." (The narrator says:) We asked: "Did the Prophet (peace and blessings be upon him) himself write this for you?" He said: "I bear witness that the Prophet (peace and blessings be upon him) wrote it for me (or had it written for me)."
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are the narrators of the Sahih, except for this man from Banu Sulaym, whom I do not know.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5192