Is water that flows from washing impurity or from istinja', and which then flows onto other parts of the body or clothing, considered ritually impure (najis) or pure (tahir)?

The water separated from ablution (istinja') is pure as long as it has not been altered by impurity. If it has been altered by impurity, then it is impure (najis). This is the madhhab of Malik, as stated by Ibn al-Hajib in "Jami' al-Ummahat". If you are certain that the dripping water has been altered by madhy (prostatic fluid) or urine, then it is obligatory to wash whatever parts of the body it has touched. If you are not certain, then nothing is required of you. One should disregard such imaginings to avoid obsessive thoughts (waswasa), because madhy or urine on the exit of the male organ is slight, and it is unlikely to alter the water. Sheikh al-Islam Ibn Taymiyyah stated that water is purifying (tahoor) as long as impurity has not mixed with it and its signs have not appeared in it, and that mere possibility does not necessitate a ruling of impurity if no signs of it appear. He supported this with the action of Umar ibn al-Khattab, may Allah be pleased with him, who performed ablution from a Christian woman's jar and prohibited questioning about water from a spout. The advice is to pour water over the place of impurity and wash it. If some of it spills onto the body, pay no attention to it unless you are certain it has been altered by impurity, and it is generally the case that water is not altered by a small amount of impurity.