بُلُوغُ الْمَرَامِ
Bulugh al-Maram 238
The Book of Prayer
عَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - قَالَ : { نَهَى رَسُولُ اَللَّهِ - صلى الله عليه وسلم -أَنْ يُصَلِّيَ اَلرَّجُلُ مُخْتَصِرًا } مُتَّفَقٌ عَلَيْهِ , وَاللَّفْظُ لِمُسْلِمٍ . [1] وَمَعْنَاهُ : أَنْ يَجْعَلَ يَدَهُ عَلَى خَاصِرَتِهِ [2]
Narrated Abu Hurairah (RA): Allah's Messenger (ﷺ) forbade keeping one's hands on one's waist while praying. [Agreed upon]. The wording is that of Muslim's. And it's meaning is to put (one's) hands on his waist.
Sunnah.com reference: Book 2, Hadith 105