Sahih Bukhari Volume 1, Book 7, Hadith 340

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنِ الْحَكَمِ، عَنْ ذَرٍّ، عَنِ ابْنِ عَبْدِ الرَّحْمَنِ بْنِ أَبْزَى، عَنْ أَبِيهِ، أَنَّهُ شَهِدَ عُمَرَ وَقَالَ لَهُ عَمَّارٌ كُنَّا فِي سَرِيَّةٍ فَأَجْنَبْنَا، وَقَالَ تَفَلَ فِيهِمَا

Narrated By 'Abdur Rahman bin Abza : That while he was in the company of 'Umar, 'Ammar said to 'Umar, "We were in a detachment and became Junub and I blew the dust off my hands (performed the rolling over the earth and prayed.)"