The fact that the Companions who observed all of his acts and words believed him so much as to sacrifice their parents prove his prophethood.
Let us think: Suppose that we have friends and some members of our family that are always with us. Is it normal for them to believe us so much as to sacrifice their and their parents’ lives when they see some of our mistakes although they love us very much? Is such a belief possible unless we are perfect and wonderful? Definitely not...
The friends of the Prophet (pbuh) that were always with him and that observed all of his acts are called the “Companions”. Would those people who observed and witnessed all of his acts closely have been so faithful to him as to sacrifice their lives if they had seen the slightest insincerity, lie or fraud of the Prophet (pbuh)? They were so faithful to him that they would fight against their parents and brothers when necessary. They were so faithful that Anas Bin Nadr was cut into pieces for his sake  and Habbab bin Arat was burnt by hot iron .
At this point, Satan can bring this question to the mind: “They may have acted like that because they were afraid of him? How can you prove it?” We can answer the question of Satan with this truth: If they had acted like that due to their fear, they would not have had to act like that after he died; Islam would have ended after his death if what Satan claims had been true. On the contrary, the fact that Islam spread all over the world after his death thanks to the Companions shows that their faith originated from the truthiness of the Prophet’s (pbuh) cause and their belief not from their fear.
That belief of the Companions made them reach the highest level after the Prophet (pbuh) in the realm of humanity. The fact that the greatest saint cannot reach the level of the smallest Companion is an indication of their belief. May Allah be pleased with them.
The fact that the Messenger of Allah saved those Companions from the savagery of the Era of Jahiliyya and elevated them to the highest level of the humanity and educated them in such a manner that they sacrificed their money, property, families, fame and lives and became guides and leaders for the most civilized communities prove his prophethood. It is the Prophet (pbuh) who trained and educated them.
We refer you to the part called “the Companions of the Prophet” for more information about the Companions, their extraordinary sacrifices and their efforts in the way of Islam.
 Bidaya, IV/32 (from Imam Ahmad), Tayalasi, Ibn Sa'd, Ibn Abi Shayba, Ibn Mundhir, Ibn Hatim, Ibn Mardawayh also mentioned it. Kanzu'l-Ummal, VIII/15, Hilya, I/121; Bayhaqi, IX/44
 Ibn Sa`d, Tabaqat: 3/165
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