I say, with Allah's guidance: Stagnant small water, which is water with a surface area less than ten cubits by ten cubits — equivalent to (25) square meters of water surface area, and a depth from which the ground does not appear when scooping — becomes impure if impurity falls into it, if it is known for certain or with predominant suspicion that impurity has fallen into it, even if the impurity does not appear in it. This is based on what Al-Bukhari narrated from Abu Huraira : 'He heard the Messenger of Allah  say: None of you should urinate in stagnant water that does not flow and then bathe in it.' The stagnant water is purified if it becomes impure by adding pure water to it until it flows. See: Al-Ta'liq Al-Mumajjad on Muwatta Muhammad 1: 269-270, Al-Sa'ayah p. 370-385, Al-Bada'i' 1: 72, and Al-Mabsut 1: 70-71.