Sunan Ibn Majah, Hadith 1441 ⓘ
It was narrated that ‘Umar bin Al-Khattab said: “The Prophet said to me: ‘When you enter upon one who is sick, tell him to pray for you, for his supplication is like the supplication of the angels.’”
حَدَّثَنَا جَعْفَرُ بْنُ مُسَافِرٍ، حَدَّثَنِي كَثِيرُ بْنُ هِشَامٍ، حَدَّثَنَا جَعْفَرُ بْنُ بُرْقَانَ، عَنْ مَيْمُونِ بْنِ مِهْرَانَ، عَنْ عُمَرَ بْنِ الْخَطَّابِ، قَالَ قَالَ لِي النَّبِيُّ ـ صلى الله عليه وسلم ـ " إِذَا دَخَلْتَ عَلَى مَرِيضٍ فَمُرْهُ أَنْ يَدْعُوَ لَكَ فَإِنَّ دُعَاءَهُ كَدُعَاءِ الْمَلاَئِكَةِ " .
also referenced under: Book 6, Hadith 1508 (In-Book) & Volume 2, Book 6, Hadith 1441 (DarusSalam)