It is narrated from Sayyiduna Umar (may Allah be pleased with him) that once I was with the Prophet (peace and blessings of Allah be upon him) in a battle. On one occasion, I swore an oath «لا وابي» when a man from behind said to me, "Do not swear oaths by the names of your forefathers." I looked and saw that it was the Prophet (peace and blessings of Allah be upon him).