What is meant by Al-Nawawi's statement, "And the Sunnah is to perform istinja' before wudu'," and how does that reconcile with Al-Fawzan's statement that some common folk believe istinja' is part of wudu' and begin with it even if they had previously performed istinja', knowing that removing impurities is obligatory?

The jurists differed regarding the validity of ablution for one who has relieved himself and then performed ablution before performing istinja (cleansing after defecation or urination). Some said it is valid due to the absence of evidence stipulating the removal of impurity before ablution. Others said it is not valid, citing the hadith of Al-Miqdad: "He should wash his private part and perform ablution." The Malikis consider istinja to be preferable to precede ablution, but not a sunnah (integral part) of it. As for the Hanbalis, their relied-upon view is that it is a condition for the validity of ablution, but there is another narration that permits it. Sheikh Ibn 'Uthaymeen said that the Prophet's action (peace be upon him) of performing istijmar (cleansing with stones) before ablution indicates preferability, not obligation, unless it was a clarification of something general. Al-Nawawi held that the sunnah is to perform istinja before ablution to avoid scholarly disagreement and to ensure the purity is not nullified. In summary: one should first remove impurity through istinja or istijmar and then perform ablution. If one performs ablution before removing the impurity and then removes it before prayer, his ablution and prayer are valid according to the majority of scholars. As for one who has not relieved himself, there is no istinja for him when performing ablution.