Sayyiduna Suwayd bin Hanzalah (may Allah be pleased with him) narrates: I came to the Prophet (peace and blessings be upon him), and Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) was with us. He encountered a people who were his enemies. The people refused to take an oath, so I stepped forward and swore that he was my brother. When we came to the Prophet (peace and blessings be upon him), I said: O Messenger of Allah! The people refused to take an oath, but I swore that he is my brother. He (peace and blessings be upon him) said: "You spoke the truth; for the Muslim is the brother of the Muslim."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20033