Musannaf Ibn Abi Shayba 3:5980

Waki' narrated to us, from al-A'mash, from Khaythama, from al-Harith ibn Qays, who said: "If you intend to do a good deed, then hasten to do it. And if the Shaytan comes to you and says, 'You are showing off,' then make it even longer."