Musnad Ahmad 624:1

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Asim ibn Bahdalah informed us, from Zirr ibn Hubaysh, who said: I went early in the morning to Safwan ibn 'Assal al-Muradi to ask him about wiping over the leather socks. He said: "What brought you?" I said: "Seeking knowledge." He said: "Shall I not give you glad tidings?" And he raised the hadith to the Messenger of Allah (peace and blessings of Allah be upon him), who said: "Indeed, the angels lower their wings for the seeker of knowledge, out of pleasure for what he is seeking." Then he mentioned the rest of the hadith.