When any one of you enters the mosque, he should observe two rak'ahs (of Nafl prayer) before sitting.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيَرْكَعْ رَكْعَتَيْنِ قَبْلَ أَنْ يَجْلِسَ  

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.  

عَنْ أَبِي قَتَادَةَ صَاحِبِ رَسُولِ اللَّهِ ﷺ قَالَ دَخَلْتُ الْمَسْجِدَ وَرَسُولُ اللَّهِ ﷺ جَالِسٌ بَيْنَ ظَهْرَانَىِ النَّاسِ قَالَ فَجَلَسْتُ فَقَالَ رَسُولُ اللَّهِ ﷺ 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  

The Apostle of Allah ﷺ owed me a debt; he paid me back and made an addition (of this). I entered the mosque and he (the Holy Prophet) said to me: Observe two rak'ahs of prayer.  

كَانَ لِي عَلَى النَّبِيِّ ﷺ دَيْنٌ فَقَضَانِي وَزَادَنِي وَدَخَلْتُ عَلَيْهِ الْمَسْجِدَ فَقَالَ لِي صَلِّ رَكْعَتَيْنِ