Muwatta Malik 159

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ كَانَ يَقُولُ مَنْ صَلَّى بِأَرْضِ فَلاَةٍ صَلَّى عَنْ يَمِينِهِ مَلَكٌ وَعَنْ شِمَالِهِ مَلَكٌ فَإِذَا أَذَّنَ وَأَقَامَ الصَّلاَةَ أَوْ أَقَامَ صَلَّى وَرَاءَهُ مِنَ الْمَلاَئِكَةِ أَمْثَالُ الْجِبَالِ ‏.‏

Yahya related to me from Malik from Yahya ibn Said that Said ibn al-Musayyab used to say, "Whoever prays on waterless, desolate land - an angel prays on his right and an angel prays on his left. When he calls both the adhan and the iqama for the prayer, or calls out the iqama, angels like mountains pray behind him

Reference

Muwatta Malik 159

In-book

Book 3, Hadith 14