Two-thirds of a Mudd were brought to the Prophet (may Allah's peace and blessings be upon him) and he began rubbing his arm

In this Hadīth ‘Abdullāh ibn Zayd (may Allah be pleased with him) informs us about the amount of water used by the Prophet (may Allah's peace and blessings be upon him) for ablution, which is two-thirds of a Mudd. This quantity of water is enough for ablution without wasting. The Hadīth also tells us that the Prophet (may Allah's peace and blessings be upon him) began to rub his arm, so as to let water reach the washed limb completely.