"If you do not help him," meaning the Prophet (ﷺ), "God has already helped him when those who disbelieved drove him out" from Mecca, "as one of two, when they were both in the cave." This is because the Quraysh gathered in Dar al-Nadwa and conspired against the Prophet; their opinion converged upon what the enemy of God, Abu Jahl, had said, which we have explained in Surah al-Anfal. God (Glorified and Exalted is He) revealed to him, so he and Abu Bakr went out at night until they arrived at the cave. The polytheists sought him but could not find him, and they continued to search. Abu Bakr had entered the cave before the Messenger of God (ﷺ) to inspect the cave and see what was in it, so that there would be no predator or snake inside, protecting the Messenger of God (ﷺ) with his own soul. Then the Messenger of God (ﷺ) entered the cave, and a dove (or pigeons) came and settled at the entrance of the cave. They both began listening to the sound of the hooves of the polytheists' mounts as they searched for them. Abu Bakr began to weep, and the Messenger of God (ﷺ) said, "What makes you weep, O Abu Bakr?" He said, "I fear that the polytheists will find you and kill you, and then God (Glorified and Exalted is He) will never be worshipped after you." The Messenger of God (ﷺ) said, "Do not grieve; indeed God is with us." Abu Bakr began wiping the tears from his cheek. "Then God sent down His tranquility upon him."

Al-Hasan said: Tranquility (al-sakina) is dignity.

Muhammad said: It is derived from stillness (al-sukun); the meaning is that He cast into his heart that by which he found peace.