Sahih Muslim · Hadith 1540
The Book of Mosques and Places of Prayer - كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
وحدثنا ابن أبي عمر، حدثنا سفيان، عن أيوب السختياني، عن ابن سيرين، عن أبي هريرة، قال قال رسول الله صلى الله عليه وسلم " إن الملائكة تصلي على أحدكم ما دام في مجلسه تقول اللهم اغفر له اللهم ارحمه ما لم يحدث وأحدكم في صلاة ما كانت الصلاة تحبسه " .
Abu Huraira reported Allah's Messenger (may peace be upon him) 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.
Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).