What is the ruling for one who prays Witr with the Imam but desires to perform an additional unit of prayer (rak'ah) to make their total prayer even before performing their actual final Witr prayer later in the night?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
If a person prays Witr with the Imam and wishes to follow the Imam in that Witr, but intends to perform their final Witr prayer later in the night, they must not conclude their prayer when the Imam concludes. Instead, after the Imam's salam, they must stand up and pray one additional rak'ah to make their total prayer congruent with the Imam's (i.e., turning the Witr into an even number of rak'ahs). This view is explicitly stated. Furthermore, it is preferred that this supplementary rak'ah is performed immediately after the Imam's salam to join the Witr prayer.
Supporting text
There is an allowance for the worshipper to either maintain the prayer as an odd number if they stand up, or to pray two rak'ahs (making the total prayer even). Regarding making the prayer even by adding one rak'ah after the Imam's salam, it is preferred. When asked about praying two units (rak'ahs) after having already performed Witr, Ahmad stated that it is permissible, provided that the final Witr prayer occurs after a period of rest (dhaj'ah).