Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Harith bin Qais that he said: When you are engaged in a worldly matter, then hasten, and when you are involved in an affair of the Hereafter, then delay as much as possible. And when you are praying and Satan comes to you and says: "You are showing off," then (even so) lengthen your prayer further.