the Most High:

“So woe to those who pray,”
“Who are heedless of their prayer,”
“Who show off,”
“And withhold even simple assistance.”

And He, the Most High, says:

“The hypocrites seek to deceive Allah, but He is deceiving them. When they stand for prayer, they stand lazily, showing off before people and remembering Allah only a little.”

As for someone who shows off in obligatory acts of worship, everyone knows how blameworthy his condition is and that Allah will punish him for not worshipping Him with sincere devotion. Allah, the Most High, says:

“They were commanded only to worship Allah with sincere devotion to Him, turning away from falsehood, to establish prayer and to give zakah. That is the upright religion.”

And He, the Most High, says:

“Indeed, We have sent down the Book to you in truth, so worship Allah with sincere devotion to Him. Surely, sincere devotion belongs to Allah alone.”

Such statements occur frequently in the Quran.

As for someone who shows off in voluntary prayers, fasting, remembrance of Allah and recitation of the Quran, no one should suppose that the consequence is merely the nullification of his deeds, leaving him with neither reward nor blame. Rather, he deserves censure and punishment for seeking renown through worship of other than Allah. These are specifically devotional acts: they are valid only when performed by a Muslim, and they may not be performed for any purpose other than drawing near to Allah. This differs from activities that benefit people, such as teaching and leading prayer; scholars disagree over the permissibility of hiring someone to perform these. And Allah knows best.