Musannaf Ibn Abi Shayba 16:584

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Husayn, from Mujahid, he said: He used to say: "Whoever lets his lower garment touch his ankles, his prayer will not be accepted." He said: And Zirr said: "Whoever lets his lower garment touch the ground, his prayer will not be accepted."