Sahih Al Bukhari 1: Chapter 3, Hadith 96

(96) Repeating one's talk thrice in order to make others understand

حَدَّثَنَا مُسَدَّدٌ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا أَبُو عَوَانَةَ، ‏‏‏‏‏‏عَنْ أَبِي بِشْرٍ، ‏‏‏‏‏‏عَنْ يُوسُفَ بْنِ مَاهَكَ، ‏‏‏‏‏‏عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، ‏‏‏‏‏‏قَالَ:‏‏‏‏ "تَخَلَّفَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي سَفَرٍ سَافَرْنَاهُ، ‏‏‏‏‏‏فَأَدْرَكَنَا وَقَدْ أَرْهَقْنَا الصَّلَاةَ صَلَاةَ الْعَصْرِ وَنَحْنُ نَتَوَضَّأُ فَجَعَلْنَا نَمْسَحُ عَلَى أَرْجُلِنَا، ‏‏‏‏‏‏فَنَادَى بِأَعْلَى صَوْتِهِ:‏‏‏‏ وَيْلٌ لِلْأَعْقَابِ مِنَ النَّارِ مَرَّتَيْنِ أَوْ ثَلَاثًا".

SAHIH HADITH:

Narrated `Abdullah bin `Amr: Once Allah's Apostle remained behind us in a journey. He joined us while we were performing ablution for the `Asr prayer which was over-due. We were just passing wet hands over our feet (not washing them properly) so the Prophet addressed us in a loud voice and said twice or thrice, Save your heels from the fire.

REFERENCE:

Sahih Al Bukhari 1: Chapter 3, Hadith 96