Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.132.

Al-Kamal ibn al-Hamam is of the opinion, “There is no authentic hadith stating that one must place the hands under the chest or below the navel. According to the Hanifiyyah, the hands are to be placed below the navel, and the Shafiyyah say below the chest. Ahmad has two narrations corresponding to these two opinions. The correct position is somewhere in the middle – to be equal.” Observes at-Tirmizhi, “Knowledgeable companions, their followers and those that came after them believed that one should put his right hand over the left during prayer, while some say above the navel and others say below the navel…” Nevertheless, there do exist hadith that the Prophet, upon whom be peace, placed his hands on his chest. Reported Hulb at-Ta’i, “I saw the Prophet, upon whom be peace, praying with his right hand over his left upon his chest above the elbow.” This is related by Ahmad and at-Tirmizhi, who grades it as hassan.

Reported Wa’il ibn Hajr, “Once when I prayed with the Prophet, upon whom be peace, he placed his right hand over his left upon his chest.” The report is recorded by Ibn Khuzaimah, who considers it as sahih, and by Abu Dawud and an-Nasa’i with the wording, “Then he put his right hand over the back of his left wrist and forearm.”