Narrated Zuhair:
"Qabus bin Abi Zabyan narrated to us, that his father narrated to him, he said: 'We said to Ibn 'Abbas: "What is the meaning of the saying of Allah the Mighty and Sublime: Allah has not made for any man two hears inside his body.? (33:4)" He said: "The Prophet of Allah (ﷺ) stood one day for Salat, then he was unsure (regarding how much he had prayed). The hypocrites who prayed with him said: 'Don't you see that he has two hearts, a heart with you and another with them?' So Allah revealed: 'Allah has not made for any man two hearts inside his body.'"
English reference: Vol. 5, Book 44, Hadith 3199
Arabic reference: Book 47, Hadith 3503