the Messenger of Allah said: "There is no marriage except with a Wali."  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ نِكَاحَ إِلاَّ بِوَلِيٍّ قَالَ وَفِي الْبَابِ عَنْ عَائِشَةَ وَابْنِ عَبَّاسٍ وَأَبِي هُرَيْرَةَ وَعِمْرَانَ بْنِ حُصَيْنٍ وَأَنَسٍ  

The Messenger of Allah said: "Whichever woman married without the permission of her Wali her marriage is invalid, her marriage is invalid, her marriage is invalid. If he entered into her, then the Mahr is for her in lieu of what he enjoyed from her private part. If they disagree, then the Sultan is the Wali for one who has no Wali."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ أَيُّمَا امْرَأَةٍ نُكِحَتْ بِغَيْرِ إِذْنِ وَلِيِّهَا فَنِكَاحُهَا بَاطِلٌ فَنِكَاحُهَا بَاطِلٌ فَنِكَاحُهَا بَاطِلٌ فَإِنْ دَخَلَ بِهَا فَلَهَا الْمَهْرُ بِمَا اسْتَحَلَّ مِنْ فَرْجِهَا فَإِنِ اشْتَجَرُوا فَالسُّلْطَانُ وَلِيُّ مَنْ لاَ وَلِيَّ لَهُ