It was narrated to me by Moḥammed bin Mūsa bin al-Mutawakkil رحمه الله, who said – it was narrated to me by ʿAlī bin al-Ḥussain al-Saʿd  Ābādi, who said – it was narrated to me by Aḥmed Abī bin ʿAbd Allāh al-Barqi, from his father, from Moḥammed ibn Ziyād, from Ābān and others:

From al-Ṣādiq Jaʿfar bin Moḥammed عليهما السلام, who said: Whoever completes his [month of] fasting with a righteous statement or a righteous action, Allāh will accept his fasts from him.
It was asked: Oh son of the Messenger of Allāh, what is the righteous statement?
He replied: The testimony of ‘there is no god by Allāh’; and the righteous action is the payment of the fast-breaking alms (al-fiṭrah).

حدثنا محمد بن موسى بن المتوكل ( رحمه الله ) ، قال : حدثنا علي بن الحسين السعد آبادي ، قال : حدثنا أحمد بن أبي عبد الله البرقي ، عن أبيه ، عن محمد ابن زياد ، عن أبان وغيره ، عن الصادق جعفر بن محمد ( عليهما السلام ) ، قال : من ختم صيامه بقول صالح أو عمل صالح تقبل الله منه صيامه . فقيل له : يا بن رسول الله ، ما القول الصالح ؟ قال : شهادة أن لا إله إلا الله ، والعمل الصالح : إخراج الفطرة .