Musnad Ahmad 30:1294

Abu Amir narrated to us, Fulayh ibn Sulayman narrated to us, from Nu’aym ibn ‘Abd Allah al-Mujmir, that he ascended to Abu Hurayrah on the roof of the mosque while he was performing ablution. He raised his arms until they reached his upper arms, then he turned to me and said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Indeed, my nation on the Day of Resurrection will be the *ghurr* (those with bright foreheads) and the *muhajjalun* (those with bright limbs) from the traces of ablution. So whoever among you is able to lengthen his *ghurrah* (brightness), let him do so." Nu’aym said: I do not know if his statement, "Whoever is able to lengthen his *ghurrah*, let him do so," is from the speech of the Messenger of Allah, peace and blessings of Allah be upon him, or from the speech of Abu Hurayrah.