Wearing clothing below the ankles is sinful in and of itself. However, it does not render the prayers of the one who does so invalid.

There is a hadîth that reports the Prophet (peace be upon him) as saying: “Indeed, Allah does not accept the prayer of a man who hangs his garments.”

This hadîth, though al-Nawawî declares it authentic in al-Majmû` (4/456) and in Riyâd al-Sâlihîn, is weak, since its chain of transmission contains Abû Ja`far who is unknown.

Moreover, its text is rejected because it goes against the broad principles of Islamic Law.

And Allah knows best.

Source: Islam Today