What is the ruling on the prayer of one who found a drop of urine after performing istinja' (ritual cleansing after defecation or urination), and the impurity of the urine was left on the clothes and the bed? Does this impurity transfer to sleepwear through sweat?

There is no obligation to purify a bed or clothes that have become impure except when intending to pray on them. Washing the impure spot suffices. It is recommended to discard and ignore obsessive thoughts (waswas). One should not prolong the stay in the restroom for answering the call of nature, and istibra' (cleansing after urination/defecation) is obligatory upon completion. If you are certain of a drop of urine and did not perform istibra' or repeat wudu', you must repeat the prayers you performed. It is not necessary to purify clothes and bedding unless you are certain that impurity has reached them, in which case washing the impure spot suffices. Your prayer with clothes whose impurity you were unaware of is valid. However, if you are certain of the impurity, prayer with them is not permissible until they are purified.