Musnad Ahmad 1023:5

Yunus ibn Muhammad narrated to us, Aban and Abd al-Samad narrated to us, Hisham narrated to us, from Yahya, from Abu Ja’far, from ‘Ata’ ibn Yasar, from one of the companions of the Prophet (peace and blessings of Allah be upon him), who said: While a man was praying and he was letting his lower garment hang down (isbal), the Prophet (peace and blessings of Allah be upon him) said to him: "Go and perform ablution." He said: So he went and performed ablution, then he came back, and the Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Go and perform ablution." He said: So he went and performed ablution, then he came back. They said: "O Messenger of Allah, why did you order him to perform ablution?" Then he remained silent regarding him. He said: "He was praying while letting his lower garment hang down, and Allah does not accept the prayer of a servant who lets his lower garment hang down."