Musnad Ahmad 25:552

Hasan narrated to us, Zuhayr narrated to us, from Qabus ibn Abi Zabyan, that his father narrated to him, saying: We said to Ibn Abbas: "What do you see regarding the saying of Allah, the Almighty and Majestic: 'Allah has not made for any man two hearts in his chest' [Al-Ahzab: 4]? What is meant by that?" He said: "The Prophet of Allah, peace and blessings of Allah be upon him, stood one day to pray. He had a passing thought, so the hypocrites who were praying with him said: 'Do you not see that he has two hearts? He has a heart with you, and a heart with them?'" So Allah, the Almighty and Majestic, revealed: "Allah has not made for any man two hearts in his chest" [Al-Ahzab: 4].