Sunan an-Nasa'i, Hadith 1839 ⓘ
It was narrated from 'Aishah: that Abu Bakr kissed the Prophet between the eyes when he had died.
أَخْبَرَنَا أَحْمَدُ بْنُ عَمْرٍو، قَالَ أَنْبَأَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ أَبَا بَكْرٍ، قَبَّلَ بَيْنَ عَيْنَىِ النَّبِيِّ صلى الله عليه وسلم وَهُوَ مَيِّتٌ .
also referenced under: Book 21, Hadith 22 (In-Book) & Volume 3, Book 21, Hadith 1840 (DarusSalam)