كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(49)
Chapter: The virtue of offering the obligatory prayers in congregation, the virtue of waiting for prayer and taking many steps towards the masjid, the virtue of walking to the masjid
(49)
باب فَضْلِ صَلاَةِ الْجَمَاعَةِ وَانْتِظَارِ الصَّلاَةِ ‏‏
Sahih Muslim 649 h

Abu Huraira reported Allah's Messenger (ﷺ) as saying:

The angels invoke blessings on everyone among you so long as he is in a place of worship with these words: O Allah! pardon him, O Allah, have mercy upon him, (and they continue to do so) as long as, he ablution (of the worshipper) is not broken, and one among you is in prayer and so long as he is detained for the prayer.
وَحَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا سُفْيَانُ، عَنْ أَيُّوبَ السَّخْتِيَانِيِّ، عَنِ ابْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ الْمَلاَئِكَةَ تُصَلِّي عَلَى أَحَدِكُمْ مَا دَامَ فِي مَجْلِسِهِ تَقُولُ اللَّهُمَّ اغْفِرْ لَهُ اللَّهُمَّ ارْحَمْهُ مَا لَمْ يُحْدِثْ وَأَحَدُكُمْ فِي صَلاَةٍ مَا كَانَتِ الصَّلاَةُ تَحْبِسُهُ ‏"‏ ‏.‏
Reference : Sahih Muslim 649h
In-book reference : Book 5, Hadith 341
USC-MSA web (English) reference : Book 4, Hadith 1396
  (deprecated numbering scheme)