Chapter 22: Oaths and Vows (Kitab Al-Aiman Wa Al-Nudhur)
Narrated Suwayd ibn Hanzalah: We went out intending (to visit) the Messenger of Allah (ﷺ) and Wa' il ibn Hujr was with us. His enemy caught him. The people desisted from swearing an oath, but I took an oath that he was my brother. So he left him. We then came to the Messenger of Allah (ﷺ), and I informed him that the people desisted from taking the oath, but I swore that he was my brother. He said: You spoke the truth: A Muslim is a brother of a Muslim.
Sunan Abi Dawud 3256