Is the Imam's audible declaration of intention (niyyah) in the Shaf' and Witr prayers valid, and what should the worshippers do if they do not know the Imam's intention, and what is the ruling on joining the Imam's prayer without knowing his intention?

Uttering the intention (niyyah) is not legislated in acts of worship; its place is in the heart. If the Imam is performing the Witr prayer, the worshiper (ma'mum) must follow him. If it is the Imam's custom to pray a certain number of rak'ahs and then perform Witr, the worshiper should follow him based on what he knows of his custom. If the worshiper doubts whether his Imam intended Witr or Qiyam (night prayer), he should intend Qiyam. If the Imam concludes the Witr prayer with the salutations, the worshiper should stand up, make his prayer an even number (by adding one rak'ah), and then perform Witr for himself, because Witr is a specific prayer that requires its own intention.