Hadith 503810
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي حَازِمِ بْنِ دِينَارٍ، عَنْ سَهْلِ بْنِ سَعْدٍ، أَنَّهُ قَالَ كَانَ النَّاسُ يُؤْمَرُونَ أَنْ يَضَعَ الرَّجُلُ الْيَدَ الْيُمْنَى عَلَى ذِرَاعِهِ الْيُسْرَى فِي الصَّلاَةِ . قَالَ أَبُو حَازِمٍ لاَ أَعْلَمُ إِلاَّ أَنَّهُ يَنْمِي ذَلِكَ .
Yahya related to me from Malik from Abu Hazim ibn Dinar that Sahl ibn Sad said, "People used to be ordered to place their right hands on their left forearms in the prayer." Abu Hazim added, "I know for sure that Sahl traces that back to the Prophet, may Allah bless him and grant him peace."
Reference Muwatta Malik 503810
USC-MSA web (English) reference : Book 9, Hadith 50