Hadith: He forbade vowing, and said: "Indeed, a vow does not bring any good; rather it is only a means to extract something from the miser."

عن ابن عمر رضي الله عنهما عن النبي صلى الله عليه وسلم «أنه نهى عن النذر، وقال: إنّ النَّذْرَ لا يأتي بخير، وإنما يُسْتَخْرَجُ به من البخيل».
[صحيح] - [متفق عليه]

Ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) forbade vowing and said: "Indeed, a vow does not bring any good; rather it is only a means to extract something from the miser.”
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbade vowing. He justified this prohibition by saying that a vow is not inherently useful, because one vows to do something that is not obligatory for him. So, it is feared that one may not fulfill his vow, and thus incur a sin. Also, by vowing, one makes an exchange with Allah, the Almighty, to worship Him in exchange for getting some benefit or getting rid of some evil. Worse than that, one may think that Allah, the Almighty, has answered his request to make him worship Him. For these and other reasons, the Prophet (may Allah's peace and blessings be upon him) forbade vowing to stay clear of these motives, and to seek Allah's generosity in return for nothing, but through hope and supplication. Finally, there is no benefit in vowing except that it makes a miserly person, who fulfills nothing but what is obligatory for him, and fulfills this vow while feeling compelled and lazy and lacking the essence of deeds, which is the righteous intention and seeking what is with Allah, the Almighty.