In the Name of Allah, most Compassionate, most Merciful

It is a very common miscalculation for Muslims today to venture into a career or work for someone without even thinking with certainty if it is haram or halal.  Why should a Muslim bother asking if it is haram or halal way to make money?  Why must he be certain that it is not halal or haram?  The answer lies in the following hadith.

Sahih Al-Bukhari Hadith 3.275 Narrated by Abu Huraira
The Prophet said, "A time will come when one will not care how one gains one's money, legally or illegally."

Sahih Al-Bukhari Hadith 3.271 Narrated by Anas
The Prophet passed by a fallen date and said, "Were it not for my doubt that this might have been given in charity, I would have eaten it." And narrated Abu Huraira the Prophet said, "I found a date-fruit fallen on my bed."

Sahih Al-Bukhari Hadith 4.347 Narrated by Khaula Al Ansariya
I heard Allah's Apostle saying, "Some people spend Allah's wealth unjustly; such people will be put in the (Hell) Fire on the Day of Resurrection."

From these hadith we can see the importance of earning money the haram or halal way and even spending it haram or halal way.  We also see the importance of keeping away from doubtful things or doubtful ways of earning and using money.  If we earn it the haram way, then it is binding on Allah to throw that person in hellfire in the day of judgement.  Should we spend our miserable 30-40 years of our life making haram money just so that we can be in hellfire for many hundreds or thousands of years, or even forever? 

Now if we examine all the topics under any hadith books, we can see that in every trade, there are guidelines by Prophet Muhammad (S).  You will see many hadith where he allowed things and prohibited things.  When we go for our work, do we even study the haram and halal first?  Do we master the subjects of Shariah first?  So the rule of thumb when earning money is to see if it is not something forbidden in Islam.  If it does involve something haram or dealing with something haram.  If there is something haram in it, it makes the whole trade of that kind as haram!  One last thing to remember is that, if the work prevents the person from doing other Islamic duties, most importantly Salah (prayers),  then that work becomes haram for that person even if the work is Halal!  Knowledge of Shariah in what you are doing is the key to earning halal money.