Sahih al-Bukhari, Volume 3, Book 33, Hadith 252 (USC-MSA) ⓘ

Narrated Abu Salama bin `Abdur-Rahman: I asked Abu Sa`id Al-Khudri, "Did you hear Allah's Messenger talking about the Night of Qadr?" He replied in the affirmative and said, "Once we were in I`tikaf with Allah's Messenger in the middle ten days of (Ramadan) and we came out of it in the morning of the twentieth, and Allah's Messengerdelivered a sermon on the 20th (of Ramadan) and said, 'I was informed (of the date) of the Night of Qadr (in my dream) but had forgotten it. So, look for it in the odd nights of the last ten nights of the month of Ramadan. I saw myself prostrating in mud and water on that night (as a sign of the Night of Qadr). So, whoever had been in I`tikaf with Allah's Messenger should return for it.' The people returned to the mosque (for I`tikaf). There was no trace of clouds in the sky. But all of a sudden a cloud came and it rained. Then the prayer was established (they stood for the prayer) and Allah's Messenger prostrated in mud and water and I saw mud over the forehead and the nose of the Prophet.

حَدَّثَنِي عَبْدُ اللَّهِ بْنُ مُنِيرٍ، سَمِعَ هَارُونَ بْنَ إِسْمَاعِيلَ، حَدَّثَنَا عَلِيُّ بْنُ الْمُبَارَكِ، قَالَ حَدَّثَنِي يَحْيَى بْنُ أَبِي كَثِيرٍ، قَالَ سَمِعْتُ أَبَا سَلَمَةَ بْنَ عَبْدِ الرَّحْمَنِ، قَالَ سَأَلْتُ أَبَا سَعِيدٍ الْخُدْرِيَّ ـ رضى الله عنه ـ قُلْتُ هَلْ سَمِعْتَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَذْكُرُ لَيْلَةَ الْقَدْرِ قَالَ نَعَمِ، اعْتَكَفْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم الْعَشْرَ الأَوْسَطَ مِنْ رَمَضَانَ ـ قَالَ ـ فَخَرَجْنَا صَبِيحَةَ عِشْرِينَ، قَالَ فَخَطَبَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم صَبِيحَةَ عِشْرِينَ فَقَالَ ‏ "‏ إِنِّي أُرِيتُ لَيْلَةَ الْقَدْرِ، وَإِنِّي نُسِّيتُهَا، فَالْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ فِي وِتْرٍ، فَإِنِّي رَأَيْتُ أَنِّي أَسْجُدُ فِي مَاءٍ وَطِينٍ، وَمَنْ كَانَ اعْتَكَفَ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَلْيَرْجِعْ ‏" ‏‏.‏ فَرَجَعَ النَّاسُ إِلَى الْمَسْجِدِ، وَمَا نَرَى فِي السَّمَاءِ قَزَعَةً ـ قَالَ ـ فَجَاءَتْ سَحَابَةٌ فَمَطَرَتْ، وَأُقِيمَتِ الصَّلاَةُ، فَسَجَدَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الطِّينِ وَالْمَاءِ، حَتَّى رَأَيْتُ الطِّينَ فِي أَرْنَبَتِهِ وَجَبْهَتِهِ‏.‏