Narrated 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 l'tikaf 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."

حَدَّثَنَا مُعَاذُ بْنُ فَضَالَةَ، حَدَّثَنَا هِشَامٌ، عَنْ يَحْيَى، عَنْ أَبِي سَلَمَةَ، قَالَ سَأَلْتُ أَبَا سَعِيدٍ وَكَانَ لِي صَدِيقًا فَقَالَ اعْتَكَفْنَا مَعَ النَّبِيِّ ﷺ 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 ﷺ فَلْيَرْجِعْ ". فَرَجَعْنَا وَمَا نَرَى فِي السَّمَاءِ قَزَعَةً، فَجَاءَتْ سَحَابَةٌ فَمَطَرَتْ حَتَّى سَالَ سَقْفُ الْمَسْجِدِ وَكَانَ مِنْ جَرِيدِ النَّخْلِ، وَأُقِيمَتِ الصَّلاَةُ، فَرَأَيْتُ رَسُولَ اللَّهِ ﷺ يَسْجُدُ فِي الْمَاءِ وَالطِّينِ، حَتَّى رَأَيْتُ أَثَرَ الطِّينِ فِي جَبْهَتِهِ.