Musnad Abu Ya'la al-Mawsili 109:319

Ibn Numayr narrated to us, Qabisah narrated to us, from Sufyan, from al-A'mash, from Abu Sufyan, from Jabir, who attributed it to the Prophet (ﷺ). He said: He used to say, "O Turner of hearts, keep my heart firm upon Your religion." We said, "O Messenger of Allah, do you fear for us, while we have believed in what you have brought to us?" He said, "Indeed, the hearts are between..." and al-A'mash gestured with two fingers.