Sahih Bukhari Volume 3, Book 32, Hadith 2017

حَدَّثَنَا مُعَاذُ بْنُ فَضَالَةَ، حَدَّثَنَا هِشَامٌ، عَنْ يَحْيَى، عَنْ أَبِي سَلَمَةَ، قَالَ سَأَلْتُ أَبَا سَعِيدٍ وَكَانَ لِي صَدِيقًا فَقَالَ اعْتَكَفْنَا مَعَ النَّبِيِّ صلى الله عليه وسلم الْعَشْرَ الأَوْسَطَ مِنْ رَمَضَانَ، فَخَرَجَ صَبِيحَةَ عِشْرِينَ، فَخَطَبَنَا وَقَالَ ‏"‏ إِنِّي أُرِيتُ لَيْلَةَ الْقَدْرِ، ثُمَّ أُنْسِيتُهَا أَوْ نُسِّيتُهَا، فَالْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ فِي الْوَتْرِ، وَإِنِّي رَأَيْتُ أَنِّي أَسْجُدُ فِي مَاءٍ وَطِينٍ، فَمَنْ كَانَ اعْتَكَفَ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَلْيَرْجِعْ ‏"‏‏.‏ فَرَجَعْنَا وَمَا نَرَى فِي السَّمَاءِ قَزَعَةً، فَجَاءَتْ سَحَابَةٌ فَمَطَرَتْ حَتَّى سَالَ سَقْفُ الْمَسْجِدِ وَكَانَ مِنْ جَرِيدِ النَّخْلِ، وَأُقِيمَتِ الصَّلاَةُ، فَرَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَسْجُدُ فِي الْمَاءِ وَالطِّينِ، حَتَّى رَأَيْتُ أَثَرَ الطِّينِ فِي جَبْهَتِهِ‏

Narrated By Abu Salama : I asked Abu Sa'id, and he was a friend of mine, (about the Night of Qadr) and he said, "We practiced Itikaf (seclusion in the mosque) in the middle third of the month of Ramadan with the Prophet. In the morning of the 20th of Ramadan, the Prophet came and addressed us and said, 'I was informed of (the date of the Night of Qadr) but I was caused to forget it; so search for it in the odd nights of the last ten nights of the month of Ramadan. (In the dream) I saw myself prostrating in mud and water (as a sign). So, whoever was in Itikaf with me should return to it with me (for another 10-day's period)', and we returned. At that time there was no sign of clouds in the sky but suddenly a cloud came and it rained till rain-water started leaking through the roof of the mosque which was made of date-palm leaf stalks. Then the prayer was established and I saw Allah's Apostle prostrating in mud and water and I saw the traces of mud on his forehead."