Mishkat al-Masabih 937
‘Abd ar-Rahman b. ‘Auf said:
God’s Messenger went out and entering among some palm trees prostrated himself so long that I was afraid God had taken his soul. I went and looked, and he raised his head and said, “What is the matter with you? I mentioned that to him, and he told me that Gabriel had come and given him the good news that God said, “If anyone invokes one blessing on you I will bless him, and if anyone greets you I will greet him.” 

Ahmad transmitted it.
وَعَنْ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ قَالَ: خَرَجَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ حَتَّى دَخَلَ نَخْلًا فَسَجَدَ فَأَطَالَ السُّجُودَ حَتَّى خَشِيتُ أَنْ يَكُونَ اللَّهُ تَعَالَى قَدْ تَوَفَّاهُ. قَالَ: فَجِئْتُ أَنْظُرُ فَرَفَعَ رَأْسَهُ فَقَالَ: «مَا لَكَ؟» فَذَكَرْتُ لَهُ ذَلِكَ. قَالَ: فَقَالَ: 
" إِنَّ جِبْرِيلَ عَلَيْهِ السَّلَام قَالَ لي: أَلا أُبَشِّرك أَن اللَّهَ عَزَّ وَجَلَّ يَقُولُ لَكَ مَنْ صَلَّى عَلَيْكَ صَلَاةً صَلَّيْتُ عَلَيْهِ وَمَنْ سَلَّمَ عَلَيْكَ سلمت عَلَيْهِ ". رَوَاهُ أَحْمد