Chapter: To say: “In sha’ Allah” while taking an oath

I went to Allah's Messenger (ﷺ) along with a group of people from (the tribe of) Al-Ash`ari, asking for mounts. The Prophet (ﷺ) said, "By Allah, I will not give you anything to ride, and I have nothing to mount you on." We stayed there as long as Allah wished, and after that, some camels were brought to the Prophet and he ordered that we be given three camels. When we set out, some of us said to others, "Allah will not bless us, as we all went to Allah's Messenger (ﷺ) asking him for mounts, and although he had sworn that he would not give us mounts, he did give us." So we returned to the Prophet; and mentioned that to him. He said, "I have not provided you with mounts, but Allah has. By Allah, Allah willing, if I ever take an oath, and then see that another is better than the first, I make expiration for my (dissolved) oath, and do what is better and make expiration."

(The Prophet) Solomon said, "Tonight I will sleep with (my) ninety wives, each of whom will get a male child who will fight for Allah's Cause." On that, his companion (Sufyan said that his companion was an angel) said to him, "Say, "If Allah will (Allah willing)." But Solomon forgot (to say it). He slept with all his wives, but none of the women gave birth to a child, except one who gave birth to a halfboy. Abu Huraira added: The Prophet (ﷺ) said, "If Solomon had said, "If Allah will" (Allah willing), he would not have been unsuccessful in his action, and would have attained what he had desired." Once Abu Huraira added: Allah apostle said, "If he had accepted."