I used to devote myself (observe i'tikaf) during these ten (nights). Then I started devoting myself in the last ten (nights). And he who desires to observe i'tikaf along with me should spend the night) at his place of i'tikaf. And I saw this night (Lailat-ul-Qadr) but I forgot it (the exact night) ; so seek it;In the last ten nights on odd numbers. I saw (the glimpses of that dream) that I was prostrating in water and mud. Abu Sa'id al-Khudri said: It rained on the twenty-first night and the water dripped (from the roof) of the mosque at the place where the Messenger of Allah ﷺ observed prayer. I looked at him and as he completed the dawn prayer, (I found) his face was wet with mud and water. (Using translation from Muslim 1167a)  

كَانَ رَسُولُ اللهِ ﷺ يُجَاوِرُ الْعَشْرَ الَّتِي فِي وَسَطِ الشَّهْرِ فَإِذَا كَانَ مِنْ حِينِ تَمْضِي عِشْرُونَ لَيْلَةً وَيَسْتَقْبِلُ إِحْدَى وَعِشْرِينَ يَرْجِعُ إِلَى مَسْكَنِهِ وَرَجَعَ مَنْ كَانَ يُجَاوِرُ مَعَهُ ثُمَّ إِنَّهُ أَقَامَ فِي شَهْرٍ جَاوَرَ فِيهِ تِلْكَ اللَّيْلَةِ الَّتِي كَانَ يَرْجِعُ فِيهَا فَخَطَبَ النَّاسَ وَأَمَرَهُمْ بِمَا شَاءَ اللهُ ثُمَّ قَالَ «إِنِّي كُنْتُ أُجَاوِرُ هَذِهِ الْعَشْرِ ثُمَّ بَدَا لِي أَنْ أُجَاوِرَ هَذِهِ الْعَشْرَ الْأَوَاخِرَ فَمَنْ كَانَ اعْتَكَفَ مَعِي فَلْيَثْبُتْ فِي مُعْتَكَفِهِ وَقَدْ رَأَيْتُ هَذِهِ اللَّيْلَةَ فَأُنْسِيتُهَا فَالْتَمِسُوهَا فِي الْعَشْرِ الْأَوَاخِرِ فِي كُلِّ وِتْرٍ وَقَدْ رَأَيْتُنِي أَسْجُدُ فِي مَاءٍ وَطِينٍ» قَالَ أَبُو سَعِيدٍ الْخُدْرِيُّ مُطِرْنَا لَيْلَةَ إِحْدَى وَعِشْرِينَ فَوَكَفَ الْمَسْجِدُ فِي مُصَلَّى رَسُولِ اللهِ ﷺ فَنَظَرْتُ إِلَيْهِ وَقَدِ انْصَرَفَ مِنْ صَلَاةِ الصُّبْحِ وَوَجْهُهُ مُبْتَلٌّ طِينًا وَمَاءً  

[Machine] The passage translates to: "It is narrated from the Prophet ﷺ that he used to observe itikaf during the last ten days of Ramadan. However, in the year he passed away, he observed itikaf for twenty days."  

عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ «يَعْتَكِفُ الْعَشْرَ الْأَوَاخِرَ مِنْ شَهْرِ رَمَضَانَ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ»  

The Prophet ﷺ used to observe i'tikaf during the last ten days of Ramadan. One year he did not observe i'tikaf. When the next year came, he observed i'tikaf for twenty nights (i.e. days). (Using translation from Abū Dāʾūd 2463)  

أَنَّ رَسُولَ اللهِ ﷺ كَانَ «يَعْتَكِفُ الْعَشْرَ الْأَوَاخِرَ مِنْ رَمَضَانَ فَسَافَرَ عَامًا فَلَمْ يَعْتَكِفْ فَلَمَّا كَانَ الْعَامُ الْمُقْبِلُ اعْتَكَفَ عِشْرِينَ»