بُلُوغُ الْمَرَامِ
Bulugh al-Maram 278
The Book of Prayer
وَعَنْ وَائِلِ بْنِ حُجْرٍ - رضى الله عنه - قَالَ : { صَلَّيْتُ مَعَ اَلنَّبِيِّ - صلى الله عليه وسلم -فَوَضَعَ يَدَهُ اَلْيُمْنَى عَلَى يَدِهِ اَلْيُسْرَى عَلَى صَدْرِهِ } أَخْرَجَهُ اِبْنُ خُزَيْمَةَ [1]
Narrated Wa'il bin Hujr (RA): I prayed with the Prophet (ﷺ) and he put his right hand on his left (and then placed them) on his chest. [Reported by Ibn Khuzaimah].
Sunnah.com reference: Book 2, Hadith 158