Hadith: A previously married woman should not be married until her verbal consent has been sought, and a virgin should not be married until her permission has been sought. They said: O Messenger of Allah, what is her permission? He said: When she remains silent

عن أبي هريرة رضي الله عنه مرفوعاً: «لا تنُكْحَ ُالأيُّم حتى تُستأمر، ولا تنكح البكر حتى تُستَأذن، قالوا: يا رسول الله، فكيف إذنها قال: أن تسكت».
[صحيح] - [متفق عليه]

Abu Hurayrah(may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "A previously married woman should not be married until her verbal consent has been sought, and a virgin should not be married until her permission has been sought." They said: "O Messenger of Allah, what is her permission?" He said: "When she remains silent."'
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Marriage is a serious contract whereby husband gains lawful access to what women guard most: private parts. By virtue of this contract, a woman becomes like a captive with her husband. Thus, the All-Wise, the Merciful, the Fair Legislator (Allah) gave her the right to choose her life partner, since she is the one who would live with him, and she is more aware of her inclinations and desires. Therefore, the Prophet (may Allah's peace and blessings be upon him) forbade marrying a previously married woman (divorced or widow) until her command is sought and she gives it. He also forbade marrying off a virgin until her permission is sought and she gives it. Since a virgin is most likely shy regarding such matters, it is sufficient to get her permission which is lighter than her command. It is also sufficient that she remains silent to indicate her consent.