Can you please quote and explain the hadith about a young man who was very active in his business and who was criticized by a Companion for not striving in the cause of Allah. Prophet Muhammad, sallallaahu ‘alayhi wa sallam, defended him by saying that there could be three reasons why he was active in his work:
1. To be self sufficient, which makes him equal to a mujahid.
2. To earn for those who depend on him, i.e. his parents/family.
3. To show off, which is considered wrong.
I heard this on YouTube, but I could not quite follow. I am also putting the link where I heard it: https://youtu.be/5hmsmP9GMzw
Also, please advise me on how to balance between work and learning about Islam so that neither of them is neglected? What should be my priority? And is it okay for a girl to delay marriage in order to get a higher degree so that I can start earning and be self-sufficient?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Ka‘b ibn ‘Ujrah, may Allah be pleased with him, said:

"A man passed by the Prophet, sallallahu ‘alayhi wa sallam, and, seeing that he was a strong active man, the Companions said, 'O Messenger of Allah, had this been in the cause of Allah (it would have been better)?' The Prophet, sallallahu ‘alayhi wa sallam, said, 'If he went out to earn a living to provide for his young children, then he is striving in the cause of Allah; if he went out to earn a living to provide for his old parents, then he is striving in the cause of Allah; if he went out to earn a living and provide for himself in order not to beg (people), then he is striving in the cause of Allah; but if he went out to show off and boast of his strength, then he is striving in the cause of the Devil.'" [At-Tabaraani]

This indicates the virtue of earning to provide for one's family. The Muslim should keenly combine acquiring knowledge, earning a living, and spending on his family and on what pleases Allah, following the example of the Companions and the righteous predecessors all.

Anas ibn Maalik, may Allah be pleased with him, said:

"Some people came to the Prophet, sallallahu ‘alayhi wa sallam, and said, 'Send with us some men to teach us the Quran and the Sunnah.' So he sent seventy men from the Ansaar (the Muslim inhabitants of Madeenah) who were called Al-Qurra' (the reciters), and my maternal uncle, Haraam, was among them. They used to recite the Quran and study its meaning at night and during the day; they used to bring water and pour it in pitchers in the mosque, then they would collect firewood and sell it, and with the sale proceeds, they would buy food for the people of As-Suffah (a place in the Prophet's mosque where the homeless and unmarried Muhajiroon [Companions who migrated from Makkah] used to dwell at the time of the Prophet, sallallahu ‘alayhi wa sallam) and the needy. The Prophet, sallallahu ‘alayhi wa sallam, sent the reciters with these people, but these (treacherous people) attacked them and killed them before they reached their destination. (While dying), they supplicated, 'O Allah, convey from us to our Prophet, sallallahu ‘alayhi wa sallam, that we have met You (in a way) that we are pleased with You and You are pleased with us.' A man attacked Haraam from behind and smote him with a spear which pierced him, whereupon Haraam said, 'By the Lord of the Ka‘bah, I have met with success.' The Messenger of Allah, sallallahu ‘alayhi wa sallam, said to his Companions, 'Your brethren have been slain, and they were saying, ‘O Allah! Convey from us to our Prophet that we have met You (in a way) that we are pleased with You and You are pleased with us.’'" [Al-Bukhaari and Muslim]

Az-Zubayr ibn Al-‘Awwaam, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, said, "If one of you were to take his rope and go to the mountain and bring a bundle of firewood on his back to sell, and thus become independent of means, that would be better for him than begging from people who may either give him something or not give him anything." [Al-Bukhaari]

The hadeeth urges the Muslims to work and earn a living. An-Nawawi wrote, "The hadeeth urges the Muslims to offer charity, work, and earn a living through lawful means."

We advise you to hasten to marriage; the Prophet, sallallahu ‘alayhi wa sallam, said, "Three (actions) are not to be delayed: the prayer when its time becomes due, the funeral when it is held, and (marriage) for the unmarried woman when she finds a suitable suitor." [At-Tirmithi and Ahmad - Al-Albaani graded it Hasan (good)]

Ibn Al-Jawzi wrote, "Some of the righteous predecessors said, 'Haste is from the devil, except for five things: serving food to a guest, preparing the deceased for burial, marrying off the virgin girl when she reaches puberty, repaying the debt when it becomes due, and repentance upon committing a sin." [Ahkaam An-Nisaa']

Allah knows best.