بُلُوغُ الْمَرَامِ
Bulugh al-Maram 293
The Book of Prayer
وَعَنْ اِبْنِ عَبَّاسٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ أَلَا وَإِنِّي نُهِيتُ أَنْ أَقْرَأَ اَلْقُرْآنَ رَاكِعًا أَوْ سَاجِدًا , فَأَمَّا اَلرُّكُوعُ فَعَظِّمُوا فِيهِ اَلرَّبَّ , وَأَمَّا اَلسُّجُودُ فَاجْتَهِدُوا فِي اَلدُّعَاءِ , فَقَمِنٌ أَنْ يُسْتَجَابَ لَكُمْ } رَوَاهُ مُسْلِمٌ [1]
Narrated Ibn 'Abbas (RA): Allah's Messenger (ﷺ) said: "I have been forbidden to recite the Qur'an while bowing or prostrating; so while in the bowing (position) glorify (the Perfection of) the Rabb (Lord), and while in the prostrating (position) be earnest in supplication, for it is fitting that your supplications may be answered." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 179