Musnad Abu Ya'la al-Mawsili 150:411

He said: I asked ‘Abd al-A‘la about the hadith of Abu Bakr al-Siddiq, and he said: "This is a mistake." And he narrated it to me, saying: Hammad narrated to us, from Ibn Abi ‘Atiq, from his father, from Abu Bakr al-Siddiq, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The siwak is a purification for the mouth and a means of pleasure for the Lord."