Leaving witr prayer with imam is something that become habit of some of the Muslims and including loss because if do not follow prayer to finish with imam then will lose big virtue in the form of reward of all night prayer.
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The Prophet sallallaahuualaihiwasallam said,
"Whosoever prayeth the night with the priest till he is finished, it is written for him (the reward of) one night (full) salat." [Hr. Ahmad]
The Prayer Witch's Prayer as the Evening Prayer Shalat
It is true, there is a hadith that zahir is ordered to make witr as the end of prayer / cover of our evening prayer. That is hadith,
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"Make the end of your prayers at night is the odd prayer (witr)" [Muttafaqqun 'alaihi]
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Ibn Hazm explains that this is just a suggestion not a necessity. He said, "Witr at the end of the night is more afdal. Whoever witir at the beginning of the night including good also, night prayer again after the witr of the law may and should not be repeated witr again (witir twice). "
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Similarly An-Nawawi explanation, he said, "Jila someone has done witr prayer then want to do sunnah prayer again in the evening, the law may and not makruh, but not repeat prayer witir again." (Muslim.or.id)
Source khazanahislamid&dakwah_sibujang