If after using toilet even after doing istibra I doubt that some urine came out while bending ( in wuzu) for whipping feets. Is wuzu again is enough, or I should do ghusl also (as written on ayatullah Sistani website). And what will be the niyat of ghusl?

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If you are sure that it is urine, then you need to clean it, and repeat wuzu.