Sunan an-Nasa'i, Volume 3, Book 21, Hadith 2043 (DarusSalam) ⓘ
It was narrated that Abu Hurairah said: "When An-Najashi died, the Prophet said: 'Pray for forgiveness for him."'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ : لَمَّا مَاتَ النَّجَاشِيُّ قَالَ النَّبِيُّ صلى الله عليه وسلم : " اسْتَغْفِرُوا لَهُ " .
also referenced under: Hadith 2041 (Reference) & Book 21, Hadith 225 (In-Book)