Musannaf Ibn Abi Shayba 33:1395

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 [to finish it quickly]. If you are in something regarding the affairs of the Hereafter, then linger as much as you are able. And if Satan comes to you while you are praying and says, "You are showing off," then increase [your devotion] and prolong [the prayer].