Is it obligatory to inquire about the purity of water dripping onto a person from a path?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
It is not obligatory to inquire about water that falls upon a person from a path because the default presumption is that the water is pure. Imam Ahmad (through the narration of Salih) stated that if the dripping occurs where ablution is not performed (a desert area, meaning not a privy), one does not need to ask about it. This is supported by the narration concerning Umar and Amr ibn al-As passing a basin, where Umar rejected being informed about whether wild beasts drink from it, stating they use it and it is used upon them, implying acceptance without inquiry.
Supporting text
If the water drips where ablution is not performed, one must wash it off, according to Salih's narration from his father.