Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: Once Sayyiduna Umar (may Allah be pleased with him) was discussing with me about making a mistake in prayer. During this, Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) came to us. He came and stayed with us. He said: I heard the Prophet (peace and blessings of Allah be upon him) say: "If a person has doubt about his prayer, then he should perform so much prayer that his doubt becomes about having added to the prayer."