Is there any Quranic verse or hadith according to which if we say our mandatory prayers (farz prayers) of 5 times regularly, (and do not say the "nafl" (which are not mandatory) prayers) and spend the rest of the time studying as a student and resting as required (for the well being of our body), is that time spent studying also considered ibadah (worship of Allah) by Allah?
Considering Ibadah (عبادة).It is better to understand its meaning first.Ibadah which we mean Worship to God has a root word in Arabic (عبد) which means a Slave or a Servant.Now a servant is bound to do what his Master says ,So ibadah does not only mean to pray 5 times or fasting but there are many ways to do it. What ever Allah Subhan O Taala has ordered us to do or not to do and then we are living in accordance to it then this is also a form of Ibadah.
Allah Subhan o Taala has said in Quran(24:37)
[Are] men whom neither commerce nor sale distracts from the remembrance of Allah and performance of prayer and giving of zakah. They fear a Day in which the hearts and eyes will [fearfully] turn about
About those people who are in commerce or sale or doing any bussiness but their hearts are not free from the remembarance of Allah All mighty so if they are earning Halal and with their hearts sticked to the remembrance of Allah then they are doing Ibadah ,By making beautiful and pure intentions to please Allah Swt in any case or in any ways it is also a form of Ibadah,and supplication is also Ibadah.the Prophet Muhammad peace be upon him said: “The supplication is the essence of worship.”(Tirmidhi)
If a person is studying with intention to seek knowledge beneficial to all or to earn halal and to spend money for Allah's cause too and and heart is filled with remembrance of Allah Swt then its also a form of Ibadah.
Apart from Mandatory prayers (Fardh),There are also Sunnah Prayers ,Sunnat Mu’akkadah prayers that the Prophet Muhammad peace be upon him observed regularly and strongly encouraged his followers to do so and Sunnat Ghair-il-Mu’akkadah prayers that the Prophet Muhammad peace be upon him did not perform regularly.Sunnah leads to Jannah.Leaving Suunah on a regular basis is also considered bad.The one who neglects an act that is an obligatory Sunnah will be punished; the one who neglects a mustahabb Sunnah will not be punished.Sunnat Muakkadah should not be left without any solid excuse. Nafl prayers are optional Prayers with which one can attain more closeness to Allah Swt.With the management of time if one is praying nafl also it is much rewarding and getting closer to Allah Swt but it is optional.
Abu Qatada, a Companion of the Messenger of Allah (ﷺ), said: I entered the mosque, when the Messenger of Allah (ﷺ) had been sitting among people, and I also sat down among them. Upon this the Messenger of Allah (ﷺ) said: What prevented you from offering two rak'ahs (of Nafl prayer) before sitting down? I said: Messenger of Allah, I saw you sitting and people sitting (around you and I, therefore, sat in your company). He (the Holy Prophet) then said: When anyone among you enters the mosque, he should not sit till he has observed two rak'ahs.(Sahih Muslim)
First of all the Niyya (intention or motive) is important as we know:
It is narrated on the authority of Amirul Mu'minin, Abu Hafs 'Umar bin al-Khattab ( may Allah be pleased with him) who said: I heard the Messenger of Allah (ﷺ) say: "Actions are (judged) by motives (niyyah), so each man will have what he intended. Thus, he whose migration (hijrah) was to Allah and His Messenger, his migration is to Allah and His Messenger; but he whose migration was for some worldly thing he might gain, or for a wife he might marry, his migration is to that for which he migrated." [Bukhari & Muslim]
so if you do some useless studies or study something which is not allowed or have the wrong intention all what you do will be in vain. As Allah said in Surat al-Furqan (25:23)
Then We shall turn . to what they had done and render it vain like scattered dust.
On the other Hand studying/learning/getting more knowledge is one of the best 'ibadat as you may find in Surat Taha (20:114) as a kind of dua'
..."O my Lord, give me more of knowledge."
There is also one Hadith that shows, if you only do what you have to do (mandatory worship) and do nothing bad you may go to Jannah (Paradise):
On the authority of Abu Abdullah Jabir bin Abdullah al-Ansaree (may Allah be pleased with him) that: A man questioned the Messenger of Allah (peace and blessings of Allah be upon him) and said, “Do you think that if I perform the obligatory prayers, fast in Ramadhan, treat as lawful that which is halal, and treat as forbidden that which is haram, and do not increase upon that [in voluntary good deeds], then I shall enter Paradise?” He (peace and blessings of Allah be upon him) replied, “Yes.” [This is the part of the hadith quoted by Imam an-Nawawi see also Sahih Muslim]
And i guess this one fits what you where looking for:
It was narrated that Abu Dharr said: "The Messenger of Allah said to me: 'O Abu Dharr! For you to come out in the morning and learn one Verse from the Book of Allah is better for you than praying one hundred Rak'ah, and for you to come out and learn a matter of knowledge, whether it is acted upon or not, is better for you than praying one thousand Rak'ah.'" [Sunan Ibn Majah ]
And if you like to read it in Arabic there is this Fatwa saying "seeking knowledge is (much) better then all kinds of optional worship طلب العلم أفضل من جميع نوافل العبادات ".
Therefore you should have as an intention to learn and study for Allah's sake or to become a positive/productive ... part to the Muslim community and work hard on it!