The Albaani Site

Al-Albaani: There is nothing to say that the prayer of a person whose lower garment is below his ankles is invalid, but he is a sinner without doubt since if he is sinful for leaving his garment below his ankles when not in prayer it is even more the case that he is sinful if he does so while praying. But the ruling to say that the prayer is null and void requires a specific text [stating that], and such a text is not found except in a hadith which An-Nawawi mentioned in Riyaadus-Saaliheen but it is a weak hadith, in it is a man called Abu Ja’far al-Madini and he is unknown.

Al-Hudaa wan-Noor, 272.