Musnad Ahmad 754
It was narrated from `Abdullah bin Yasar that `Amr bin Huraith visited al-Hasan bin `Ali (رضي الله عنهما) [when he was sick]. `Ali said to him:
Are you visiting al-Hasan [during his sickness] when you feel what you feel? He said to him: You are not my Lord, to direct my heart as you will. ‘Ali (رضي الله عنه) said: That does not prevent us from giving you advice. I heard the Messenger of Allah (ﷺ)  say: `There is no Muslim who visits his sick brother, but Allah will send to him seventy thousand angels who will send blessings upon him from whatever hour of the day it is until evening comes, and from whatever hour of the night it is until morning comes. `Amr said to him: What do you say about walking in front of or behind the bier at a funeral? `Ali (رضي الله عنه) said. The superiority of walking behind it over walking in front of it is like the superiority of offering a prescribed prayer in congregation over offering the prayer alone. `Amr said: But I saw Abu Bakr und ‘Umar  walking in front of the bier, Ali (رضي الله عنه) said; They did not like to embarrass the people.
حَدَّثَنَا يَزِيدُ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ يَعْلَى بْنِ عَطَاءٍ، عَنْ عَبْدِ اللَّهِ بْنِ يَسَارٍ، أَنَّ عَمْرَو بْنَ حُرَيْثٍ، عَادَ الْحَسَنَ بْنَ عَلِيٍّ رَضِيَ اللَّهُ عَنْهُ فَقَالَ لَهُ عَلِيٌّ أَتَعُودُ الْحَسَنَ وَفِي نَفْسِكَ مَا فِيهَا فَقَالَ لَهُ عَمْرٌو إِنَّكَ لَسْتَ بِرَبِّي فَتَصْرِفَ قَلْبِي حَيْثُ شِئْتَ قَالَ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ أَمَا إِنَّ ذَلِكَ لَا يَمْنَعُنَا أَنْ نُؤَدِّيَ إِلَيْكَ النَّصِيحَةَ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ مَا مِنْ مُسْلِمٍ عَادَ أَخَاهُ إِلَّا ابْتَعَثَ اللَّهُ لَهُ سَبْعِينَ أَلْفَ مَلَكٍ يُصَلُّونَ عَلَيْهِ مِنْ أَيِّ سَاعَاتِ النَّهَارِ كَانَ حَتَّى يُمْسِيَ وَمِنْ أَيِّ سَاعَاتِ اللَّيْلِ كَانَ حَتَّى يُصْبِحَ قَالَ لَهُ عَمْرٌو وَكَيْفَ تَقُولُ فِي الْمَشْيِ مَعَ الْجِنَازَةِ بَيْنَ يَدَيْهَا أَوْ خَلْفَهَا فَقَالَ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ إِنَّ فَضْلَ الْمَشْيِ مِنْ خَلْفِهَا عَلَى بَيْنِ يَدَيْهَا كَفَضْلِ صَلَاةِ الْمَكْتُوبَةِ فِي جَمَاعَةٍ عَلَى الْوَحْدَةِ قَالَ عَمْرٌو فَإِنِّي رَأَيْتُ أَبَا بَكْرٍ وَعُمَرَ رَضِيَ اللَّهُ عَنْهُمَا يَمْشِيَانِ أَمَامَ الْجِنَازَةِ قَالَ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ إِنَّهُمَا إِنَّمَا كَرِهَا أَنْ يُحْرِجَا النَّاسَ‏.‏