The Prophet (ﷺ) forbade vowing and said, "In fact, vowing does not prevent anything, but it makes a miser to spend his property."
Sahih al-Bukhari Book 82, Hadith 14
← Previous Hadith There was a man who fought most bravely of all the Muslims on behalf of the Muslims in a battle (Ghazwa) in the company of the Prophet. The Prophet (ﷺ) looked at him and said. "If anyone would like to see a man from the people of the Fire, let him look at this (brave man)." On that, a man from the People (Muslims) followed him, and he was in that state i.e., fighting fiercely against the pagans till he was wounded, and then he hastened to end his life by placing his sword between his breasts (and pressed it with great force) till it came out between his shoulders. Then the man (who was watching that person) went quickly to the Prophet (ﷺ) and said, "I testify that you are Allah's Messenger (ﷺ)!" The Prophet (ﷺ) asked him, "Why do you say that?" He said, "You said about so-and-so, 'If anyone would like to see a man from the people of the Fire, he should look at him.' He fought most bravely of all of us on behalf of the Muslims and I knew that he would not die as a Muslim (Martyr). So when he got wounded, he hastened to die and committed suicide." There-upon the Prophet (ﷺ) said, "A man may do the deeds of the people of the Fire while in fact he is one of the people of Paradise, and he may do the deeds of the people of Paradise while in fact he belongs to the people of Fire, and verily, (the rewards of) the deeds are decided by the last actions (deeds)".

Sahih al-Bukhari Book 82, Hadith 13 Next Hadith → The Prophet (ﷺ) said (that Allah said), "Vowing does not bring to the son of Adam anything I have not already written in his fate, but vowing is imposed on him by way of fore ordainment. Through vowing I make a miser spend of his wealth."

Sahih al-Bukhari Book 82, Hadith 15