Al-Matalib al-'Aliyah 3220 — The rest of the warning against showing

It is narrated from Muhammad bin Wasi' (may Allah have mercy on him) that Uways al-Qarani (may Allah have mercy on him) saw a man who, during his prayer, (in agitation) would at times stand up and at times sit down. He asked him: "What is the matter with you?" He said: "I stand up for prayer, and Satan comes and whispers that you are showing off, upon which I (in fear) sit down. Then my soul draws me back to prayer and I stand up, but he comes again and says that you are showing off, so I sit down again." He asked him: "If you were in seclusion and alone, would you still pray this same prayer?" He replied: "Yes." He said: "Then complete your prayer, for you are certainly not a show-off."
Imam Ahmad bin Hanbal (may Allah have mercy on him) narrated it in Kitab al-Zuhd.