Sahih Muslim Book 27, Hadith 17
We walked on foot and came to Allah's Apostle (may peace he upon him) asking him to provide us with mounts. The rest of the hadith is the same.
We came to Allah's Messenger (ﷺ) requesting him to provide us with riding camels. He (the Holy Prophet) said: There is nothing with me with which I should equip you. By Allah, I would not provide you with (riding camels). Then Allah's Messenger (ﷺ) sent to us three camels with spotted bumps. We said: We came to Allah's Messenger (ﷺ) asking him to equip us with riding animals. He took an oath that he could not equip us. We came to him and informed him. He said: By Allah, I do not take an oath, but when I find the other thing better than that, I do that which is better.
A person sat late in the night with Allah's Apostle (ﷺ), and then came to his family and found that his children had gone to sleep. His wife brought food for him. but he took an oath that he would not eat because of his children (having gone to sleep without food) He then gave precedence (of breaking the vow and then expiating it) and ate the food He then came to Allah s Messenger (ﷺ) and made mention of that to him, whereupon Allah's Messenger (may peace he upon him) said: He who took an oath and (later on) found something better than that should do that, and expiate for (breaking) his vow.