مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 798
Prayer
The Nature of Prayer - Section 1
وَعَن سهل بن سعد قَالَ: كَانَ النَّاسُ يُؤْمَرُونَ أَنْ يَضَعَ الرَّجُلُ الْيَدَ الْيُمْنَى عَلَى ذِرَاعِهِ الْيُسْرَى فِي الصَّلَاةِ. رَوَاهُ البُخَارِيّ
Sahl b. Sa'd said that the people received command that a man should place the right hand on the left forearm in prayer. Bukhari transmitted it.
Mishkat al-Masabih, 798صَحِيح (الألباني)