Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.015C.

Rope used for hanging clothes with impurities on them may afterwards be used for hanging pure clothes.

If a liquid falls on a person and he does not know if it was water or urine, he need not inquire about it. If he does inquire, the one who is asked need not answer him even if he knows that the liquid is impure. In that case, the person need not wash his clothes.

If a person finds something moist on his body or clothes at night, and he does not know what it is, he need not smell it to discover what it might be. It is related that ‘Umar passed by a gutter (and got wet). ‘Umar’s companion asked the owner of the gutter if the water was pure or impure. ‘Umar told the owner not to answer the question, and went on his way.

Clothes that have street mud on them need not be washed. Reported Kamyal ibn Ziyad, “I saw ‘Ali wading through the mud, after which he entered the mosque and prayed without washing his legs.”

If a person finishes his prayer and sees some impurities on his clothes or body of which he was not previously aware, or he was aware of them but forgot about them, or he did not forget about them but he was not able to remove them, then his prayer is still valid and he need not repeat it. This opinion is supported by Allah’s statement, “And there is no sin for you in the mistakes you make unintentionally.” (al-Ahzab 5). Many of the companions and those of the following generation gave this legal verdict.

If a person can not determine what part of his clothes contain the impurity, he should wash the whole garment. This is based on the axiom, “If an obligation cannot be fulfilled except by performing another related act, then that act also becomes obligatory.”

If a person mixes his pure clothes with his impure clothes (and gets confused between them), he should investigate the matter and pray once in one of the clothes. This is similar to the question of the exact direction of the qiblah. It does not matter if the proportion of pure clothes was large or small.