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Thabit ibn 'Ubayd said, "I came to a group which included 'Abdullah ibn 'Umar who said, 'When you greet, make it clearly audible, for it is a greeting from Allah containing great blessing.:
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا خَلاَّدُ بْنُ يَحْيَى، قَالَ: حَدَّثَنَا مِسْعَرٌ، عَنْ ثَابِتِ بْنِ عُبَيْدٍ قَالَ: أَتَيْتُ مَجْلِسًا فِيهِ عَبْدُ اللهِ بْنُ عُمَرَ، فَقَالَ: إِذَا سَلَّمْتَ فَأَسْمِعْ، فَإِنَّهَا تَحِيَّةٌ مِنْ عِنْدِ اللهِ مُبَارَكَةً طَيْبَةً.
sunnah.com