كتاب الجهاد والسير
Fighting for the Cause of Allah (Jihaad)
Chapter: The dust which falls on head in Allah's Cause
باب مَسْحِ الْغُبَارِ عَنِ النَّاسِ، فِي السَّبِيلِ
Sahih al-Bukhari 2812

Narrated `Ikrima:

that Ibn `Abbas told him and `Ali bin `Abdullah to go to Abu Sa`id and listen to some of his narrations; So they both went (and saw) Abu Sa`id and his brother irrigating a garden belonging to them. When he saw them, he came up to them and sat down with his legs drawn up and wrapped in his garment and said, "(During the construction of the mosque of the Prophet) we carried the adobe of the mosque, one brick at a time while `Ammar used to carry two at a time. The Prophet (ﷺ) passed by `Ammar and removed the dust off his head and said, "May Allah be merciful to `Ammar. He will be killed by a rebellious aggressive group. `Ammar will invite them to (obey) Allah and they will invite him to the (Hell) fire."

حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى، أَخْبَرَنَا عَبْدُ الْوَهَّابِ، حَدَّثَنَا خَالِدٌ، عَنْ عِكْرِمَةَ، أَنَّ ابْنَ عَبَّاسٍ، قَالَ لَهُ وَلِعَلِيِّ بْنِ عَبْدِ اللَّهِ ائْتِيَا أَبَا سَعِيدٍ فَاسْمَعَا مِنْ حَدِيثِهِ‏.‏ فَأَتَيْنَاهُ وَهُوَ وَأَخُوهُ فِي حَائِطٍ لَهُمَا يَسْقِيَانِهِ، فَلَمَّا رَآنَا جَاءَ فَاحْتَبَى وَجَلَسَ فَقَالَ كُنَّا نَنْقُلُ لَبِنَ الْمَسْجِدِ لَبِنَةً لَبِنَةً، وَكَانَ عَمَّارٌ يَنْقُلُ لَبِنَتَيْنِ لَبِنَتَيْنِ، فَمَرَّ بِهِ النَّبِيُّ صلى الله عليه وسلم وَمَسَحَ عَنْ رَأْسِهِ الْغُبَارَ وَقَالَ ‏ "‏ وَيْحَ عَمَّارٍ، تَقْتُلُهُ الْفِئَةُ الْبَاغِيَةُ، عَمَّارٌ يَدْعُوهُمْ إِلَى اللَّهِ وَيَدْعُونَهُ إِلَى النَّارِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 2812
In-book reference : Book 56, Hadith 28
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 67
  (deprecated numbering scheme)