Surah al-baqarah 2/238
“Maintain with care the (obligatory) Prayers and (in particular) the middle prayer and stand before Allaah, devoutly obedient.”
Sahih Al-Bukhari Hadith Narrated by Talha bin Ubaidullah
A man from Najd with unkempt hair came to Allah's Messenger (saws) and we heard his loud voice but could not understand what he was saying, till he came near and then we came to know that he was asking about Islam. Allah's Messenger (saws) said, "You have to offer prayers perfectly five times in a day and night (24 hours)." The man asked, "Is there any more (praying)?" Allah's Messenger (saws) replied, "No, but if you want to offer the Nawafil prayers (you can)." Allah's Messenger (saws) further said to him: "You have to observe fasts during the month of Ramadan." The man asked, "Is there any more fasting?" Allah's Messenger (saws) replied, "No, but if you want to observe the Nawafil fasts (you can.)" Then Allah's Messenger (saws) further said to him, "You have to pay the Zakat (obligatory charity)." The man asked, "Is there any thing other than the Zakat for me to pay?" Allah's Messenger (saws) replied, "No, unless you want to give alms of your own." And then that man retreated saying, "By Allah! I will neither do less nor more than this (the obligatory duties)!." Allah's Messenger (saws) said, "If what he said is true, then he will be successful (i.e. he will be granted Paradise)."
Narrated in Muslim
Umm Habibah (May Allah be pleased with her)
the Mother of the Believers reported:
I heard the Messenger of Allah (PBUH) saying, "A house will be built in Jannah for every Muslim who offers twelve Rak`ah of optional Salat other than the obligatory Salat in a day and a night (to seek the Pleasure of Allah).''
Narrated in Abo-dawood
The prophet (Salla-llaahu ‘alayhi wasallam) is reported to have said: “The first thing about which the people will be called to account out of their actions on the Day of Judgment is Prayer. Our Lord, the Exalted, will say to the angels - though He knows better: “Look into the prayer of My servant and see whether he has offered it perfectly or imperfectly.” If it is perfect, that will be recorded perfect. If it is defective, He will say: “See if there are some optional prayers offered by My servant.” If there are voluntary prayers to his credit, He will say: “Compensate the obligatory prayers with the voluntary prayers for My servant.” Then all the actions will be considered similarly.”
So pondering upon the above, there is no sin on the person who does not perform the non-obligatory prayers (sunnah or nawafil) the only thing that can be said about this person is that they would be missing out on a great reward.
And Allah SWT knows best.