The Prophet, peace and blessings of Allah be upon him, when he stood up to pray, used to place his right hand on his left hand (Recorded in Muslim no. 401) and place them on his chest (Recorded in Abu Dawud no. 759).
The Messenger of Allah, peace and blessing of Allah be upon him said:
“We Prophets were commanded… to place our right hands on the left in prayer.” (Recorded in at-Tabarani no.11485)
Imam Ahmad, may Allah have mercy on him, was asked about the meaning of placing one hand on top of the other when standing in prayer. He said: “It is humility before the Almighty.” (Recorded in Al-Khushu fi-‘l Salah by Ibn Rajab p21)
Ibn Hajr, may Allah have mercy on him, said: “The ulama said: the meaning of this posture is that it is the attitude of the humble petitioner, it is more likely to prevent fidgeting, and it more conducive to khushu.” (Fath al-Baari 2/224)