If it is sunnah, it is not obligatory!

Is it obligatory to performs sunnah muakkada? I understand about huge reward associated with Sunnah Mukkadah, but will it be sin to leave it without any excuse? I am specifically consern about performing tarawee Irregularly and also about other Sunnah mukkada. Can you kindly provide you answer with references from ruling of 4 madhab and hadith?

Prayer | July 9, 2015 1:09 am
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A female prays some raka’as of taraweeh in Ramadan and sleeps at mid night with intention to wake up at 3 AM and pray rest of taraweeh and witr. The fajr is at 4AM. When she wakes up she finds herself menstruating.My question is after she purifies herself, does she need to make up witr prayer?
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i complete my witir prayer with imam after trawi prayer and then i go for KIAM UL LAIL that time also imam praying witir prayer. kindly advice me. i saw some people stand up when imam praying WITIR after KIAM UL LAIL.