I know he was a merchant prior to becoming prophet. After he became a prophet, was he still a merchant? Did he have a day job or something like that? Did he receive salary for being a prophet? Otherwise, did he have some other form of income to feed/provide for his (presumably rather large) family?
After responding to Allah's request to tell the people about the message of Islam. He dedicated all of his time for Dawah and taking care of newly converts, i.e., checking on them, teaching them about Allah and Quran, organizing groups to migrate to Madinah and Abyssinia, etc... And during this time, Khadija (his first wife) was an owner of a really successful business who the prophet used to take care of.
After moving to Madinah and the death of his wife Khadija, his full time job was to create an Islamic state (the first mosque, treaty with Jews, Aansar share a house with muhajir), which would never let him start a business or a job.
Even-though the prophet didn't have a salary per se. but he used to get some income from these sources:
Gifts: People used to give him gifts which he always accept (Sunnah in Islam is never to reject a gift).
Khumus: war booties (For more information check Why does the Shia and Sunni viewpoint differ on the concept of Khums? And what are those differences?)
Sadaqa: Although many people used to donate money, sheep, and other valuable goods. He would immediately distribute them between the poor and needy.
'A'isha reported: Never had the family of Muhammad (ﷺ) eaten to the fill since their, arrival in Medina with the bread of wheat for three successive nights until his (Holy Prophet's) death. Sahih Muslim
Also, this long hadith in Bukhary which shows how the prophet used to give Sadaqa to needy.
In the name of God
By reviewing the life of the prophet of Islam (p) we can easily understand that he (p)always work to gain daily portion of lawful food for himself and his family. He (p) trade or merchant before becoming prophet in Mecca. After coming to Medina he always farm because of the condition of the city of Medina.
In his perspective (p) working or having a job is very important and he (p) introduced some ways to gain a daily portion of lawful food such as:
trust in God, offering a petition to god or pray, marriage, repentance and so on.
There are many Hadiths that show the important of working for the prophet (p): Ibin Abass said when the prophet (p) paid attention to someone he (p) first asked what is your job? If he answered, I don't have one, he became pained.
He (p) also believe that for protecting our religion (Islam) and the tenets of religious doctrine we should work and have job because if we don't have job, we will use our tenets of religious doctrine in the wrong or bad way (1). So we can understand by reading the last point he (p) never gave salary for being prophet and in his view, this subject is forbidden. Also by reviewing lives of other prophets (p) we see that they didn't give any money or some thing like this for being prophet.
reference: http://www.iec-md.org/ahlulbayt/rasul_akram_maeyshat_jabbaari.html (in Persian)