Al-Hârith b. Qays – Allah have mercy on him – said:

When intending to do something good, do not delay it until tomorrow. When involved in something to do with the hereafter, keep yourself involved as long as you can. When involved in some worldly matter, put your mind to it (so as to get it done). And if you are praying and Shaytân whispers, ‘You are showing off,’ then make your prayer even longer.

Ibn Al-Mubârak in Al-Zuhd wa Al-Rqâ’iq Vol.1 p126

Al-Hârith b. Qays was a student of the Companions ‘Alî and Ibn Mas’ûd – Allah be pleased with them. He was a scholar and devout worshipper.

The last bit of advice about prayer serves to repel one of Shaytân’s plots, which is to turn people away from worshipping Allah by making them feel they are showing off if they try to perfect their worship.