(13) The Book of Forgetfulness (In Prayer)
كتاب السهو
(18) Chapter: Weeping during prayer
(18) باب الْبُكَاءِ فِي الصَّلاَةِ
Sunan an-Nasa'i 1214 
It was narrated from Mutarrif that his father said:
"I came to the Prophet (ﷺ) when he was praying, and there was a sound coming from his chest like the sound of water boiling," meaning, he was weeping.
أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ حَمَّادِ بْنِ سَلَمَةَ، عَنْ ثَابِتٍ الْبُنَانِيِّ، عَنْ مُطَرِّفٍ، عَنْ أَبِيهِ، قَالَ أَتَيْتُ النَّبِيَّ صلى الله عليه وسلم وَهُوَ يُصَلِّي وَلِجَوْفِهِ أَزِيزٌ كَأَزِيزِ الْمِرْجَلِ يَعْنِي يَبْكِي ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 1214
In-book reference : Book 13, Hadith 36
English translation : Vol. 2, Book 13, Hadith 1215