Zakāt al-Fiṭr: What Is a Sāʿ? What Is a Mudd?
In the Name of Allāh, the Ever Merciful, the Bestower of Mercy
...This is the mudd that the Prophet (may Allāh raise his rank and grant him peace) was speaking about when he prayed, "O Allāh! Put blessings in our saa' and our mudd!" This was collected by the two shaykhs (al-Bukhārī and Muslim). Further, he (may Allāh raise his rank and grant him peace) used to make wuḍūʿ using this mudd, and he used to take a bath using a saa'.
Read the discussion on salafitalk.net