Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 80

وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا دَخَلَ الْمَسْجِدَ لَمْ يَرْفَعْ أَحَدٌ رَأْسَهُ غَيْرُ أَبِي بَكْرٍ وَعُمَرَ كَانَا يَتَبَسَّمَانِ إِلَيْهِ وَيَتَبَسَّمُ إِلَيْهِمَا رَوَاهُ التِّرْمِذِيُّ. وَقَالَ: هَذَا حَدِيثٌ غَرِيبٌ

Anas told that when God’s messenger entered the mosque no one but Abu Bakr and `Umar raised his head. They would smile to him and he would smile to them. Tirmidhi transmitted it, saying this is a gharib tradition.