Mishkat al-Masabih 2248
‘Umar b. al-Khattab said:
I asked the Prophet’s permission to perform an ‘umra and he gave me permission, saying, “Include me in your supplication, little brother, and do not forget me.” Then he said a word to me for which I would not be willing to take the world in exchange. 

Abu Dawud and Tirmidhi transmitted it, Tirmidhi’s version ending with “do not forget me.”
وَعَنْ عُمَرَ بْنِ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ قَالَ اسْتَأْذَنْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي الْعُمْرَةِ فَأَذِنَ لِي وَقَالَ: «أَشْرِكْنَا يَا أُخَيُّ فِي دُعَائِكَ وَلَا تَنْسَنَا» . فَقَالَ كَلِمَةً مَا يَسُرُّنِي أَنَّ لِيَ بِهَا الدُّنْيَا. رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَانْتَهَتْ رِوَايَتُهُ عِنْدَ قَوْلِهِ «لَا تنسنا»