كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(11)
Chapter: It is recommended to greet the masjid by praying two rak`ah, and it is disliked to sit before praying these two rak`ah, and this is prescribed at all times
(11)
باب اسْتِحْبَابِ تَحِيَّةِ الْمَسْجِدِ بِرَكْعَتَيْنِ وَكَرَاهَةِ الْجُلُوسِ قَبْلَ صَلاَتِهِمَا وَأَنَّهَا مَشْرُوعَةٌ فِي جَمِيعِ الأَوْقَاتِ
Sahih Muslim 714 b

Abu Qatada, a Companion of the Messenger of Allah (ﷺ), said:

I entered the mosque, when the Messenger of Allah (ﷺ) had been sitting among people, and I also sat down among them. Upon this the Messenger of Allah (ﷺ) said: What prevented you from offering two rak'ahs (of Nafl prayer) before sitting down? I said: Messenger of Allah, I saw you sitting and people sitting (around you and I, therefore, sat in your company). He (the Holy Prophet) then said: When anyone among you enters the mosque, he should not sit till he has observed two rak'ahs.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حُسَيْنُ بْنُ عَلِيٍّ، عَنْ زَائِدَةَ، قَالَ حَدَّثَنِي عَمْرُو بْنُ يَحْيَى الأَنْصَارِيُّ، حَدَّثَنِي مُحَمَّدُ بْنُ يَحْيَى بْنِ حَبَّانَ، عَنْ عَمْرِو بْنِ سُلَيْمِ بْنِ خَلْدَةَ الأَنْصَارِيِّ، عَنْ أَبِي قَتَادَةَ، صَاحِبِ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ دَخَلْتُ الْمَسْجِدَ وَرَسُولُ اللَّهِ صلى الله عليه وسلم جَالِسٌ بَيْنَ ظَهْرَانَىِ النَّاسِ - قَالَ - فَجَلَسْتُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَا مَنَعَكَ أَنْ تَرْكَعَ رَكْعَتَيْنِ قَبْلَ أَنْ تَجْلِسَ ‏"‏ ‏.‏ قَالَ فَقُلْتُ يَا رَسُولَ اللَّهِ رَأَيْتُكَ جَالِسًا وَالنَّاسُ جُلُوسٌ ‏.‏ قَالَ ‏"‏ فَإِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلاَ يَجْلِسْ حَتَّى يَرْكَعَ رَكْعَتَيْنِ ‏"‏ ‏.‏
Reference : Sahih Muslim 714b
In-book reference : Book 6, Hadith 85
USC-MSA web (English) reference : Book 4, Hadith 1541
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