Sunan an-Nasa'i, Book 13, Hadith 36 (In-Book) ⓘ
It was narrated from Mutarrif that his father said: "I came to the Prophet 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 .
also referenced under: Hadith 1214 (Reference) & Volume 2, Book 13, Hadith 1215 (DarusSalam)