(40) باب فَضْلِ لَيْلَةِ الْقَدْرِ وَالْحَثِّ عَلَى طَلَبِهَا وَبَيَانِ مَحَلِّهَا وَأَرْجَى أَوْقَاتِ طَلَبِهَا
Abu Sa'id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) devoted (himself to prayer) in the middle (ten nights) of Ramadan. The rest of the hadith is the same except for these words:
" That he adhered to his place of i'tikaf and his forehead was besmeared with mud and water."
Reference: Sahih Muslim 1167 b
In-book reference: Book 13, Hadith 276
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2626