SHAQQU ‘S-SADR. شق الصدر
Ljt. “The splitting open of the heart.” Anas relates that “the Angel Gabriel came to the Prophet, when he was playing with boys, and took hold of him, and laid him on the ground, and split open his heart, and brought out a little bag of blood, and said to Muhammad, ‘This is the devil’s part of you.’ After this, Gabriel washed, the Prophet’s heart with zamzam water, then sewed it up and replaced it. Then the boys who were with the Prophet came running to his nurse, saying, ‘Verily Muhammad is killed.!’ Anas also says that he had seen the marks of the sewing in the Prophet’s breast.” (Mishkat, book xxiv. ch. vi.)
According to the commentators al-Baizawi, al-Kamalan, and Husain, the first verse of the xcivth Surah of the Qur’an refers to this event: “Have we not opened thy breast for thee, and taken off from thee thy burden, which galled thy back?” [MUHAMMAD.]

Based on Hughes, Dictionary of Islam