His prayer will be accepted

Sahih Bukhari 2.253 :

Narrated 'Ubada bin As−Samit: The Prophet "Whoever gets up at night and says: −− 'La ilaha il−lallah Wahdahu la Sharika lahu Lahu−l−mulk, waLahu−l−hamd wahuwa 'ala kullishai'in Qadir. Al hamdu lil−lahi wa subhanal−lahi wa la−ilaha il−lal−lah wa−l−lahu akbar wa la hawla Wala Quwata il−la−bil−lah.' (None has the right to be worshipped but Allah. He is the Only One and has no partners . For Him is the Kingdom and all the praises are due for Him. He is Omnipotent. All the praises are for Allah. All the glories are for Allah. And none has the right to be worshipped but Allah, And Allah is Great And there is neither Might nor Power Except with Allah). And then says: −− Allahumma, Ighfir li (O Allah! Forgive me). Or invokes (Allah), he will be responded to and if he performs ablution (and prays), his prayer will be accepted."