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 النَّبِيَّ ﷺ كَانَ «يَعْتَكِفُ الْعَشْرَ مِنْ رَمَضَانَ فَسَافَرَ عَامًا فَلَمْ يَعْتَكِفْ فَلَمَّا كَانَ قَابِلٌ اعْتَكَفَ عِشْرِينَ لَيْلَةً الِاجْتِهَادُ فِي الْعَشْرِ الْأَوَاخِرِ وَالْتِمَاسُ لَيْلَةِ الْقَدْرِ فِيهَا»  

'A'isha (Allah be pleased with her) reported that Messenger of Allah ﷺ used to exert himself in devotion during the last ten nights to a greater extent than at any other time. (Using translation from Muslim 1175)  

عَائِشَةُ كَانَ رَسُولُ اللهِ ﷺ «يَجْتَهِدُ فِي الْعَشْرِ مَا لَا يَجْتَهِدُ فِي غَيْرِهَا»  

"Aishah may Allah (SWT) be pleased with her, said: ' When the last ten nights of Ramadan began, the Messenger of Allah ﷺ stayed up at night (for prayer) and he woke his family up and tightened his waist-wrap.'" (Using translation from Nasāʾī 1639)  

عَائِشَةُ «كَانَتْ إِذَا دَخَلَتِ الْعَشْرُ أَحْيَا رَسُولُ اللهِ ﷺ اللَّيْلَ وَأَيْقَظَ أَهْلَهُ وَشَدَّ الْمِئْزَرَ»  

[Machine] The Messenger of Allah, ﷺ , said, "I was shown the Night of Decree, then my family woke me up and I forgot it, so seek it in the odd nights of the last ten days."  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «أُرِيتُ لَيْلَةَ الْقَدْرِ ثُمَّ أَيْقَظَنِي أَهْلِي فَنَسِيتُهَا فَالْتَمِسُوهَا فِي الْعَشْرِ الْغَوَابِرِ»  

[Machine] The Messenger of Allah ﷺ said, "I was shown the Night of Qadr (Decree) and then I forgot it. So, seek it in the last ten nights of Ramadan."  

قَالَ رَسُولُ اللهِ ﷺ «أُرِيتُ لَيْلَةَ الْقَدْرِ ثُمَّ نُسِّيتُهَا فَالْتَمِسُوهَا فِي الْعَشْرِ الْغَوَابِرِ»