All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Dear inquirer!
Know that both cases of keeping hands on one's chest and to one's sides while praying are permissible.
However, the majority of Muslim scholars are of the view that keeping the hands on one's chest demonstrates the Sunnah of the Prophet for the Hadeeth reported from Wa'il Ibn Hujr which indicates that “the Prophet put his right hand over his left one while praying.” [Muslim]
As for the second state, which is leaving the hands on one's side, it is the opinion of the Maliki School of jurisprudence, but the evidence supports the first opinion.
Allah knows best.