Musannaf Ibn Abi Shayba 33:703

Abu Mu'awiyah narrated to us, from al-A'mash, from Khaythama, from al-Harith ibn Qays, who said: If you are in something regarding the affairs of the world, then aim for it (hasten). And if you are in something regarding the affairs of the Hereafter, then pause as much as you are able. And if the devil comes to you while you are praying and says, "You are showing off," then increase (your prayer) and lengthen it.