صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب صلاة التطوع قبل الصلوات المكتوبات وبعدهنChapters on Voluntary Prayers before and after Obligatory Prayers
(527) بَابُ ذِكْرِ الْخَبَرِ الْمُفَسِّرِ لِأَمْرِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِأَنْ تُصَلَّى الرَّكْعَتَانِ بَعْدَ الْمَغْرِبِ فِي الْبُيُوتِThe chapter explaining the Prophet's command to pray the two units after Maghrib at home
Hadith Q1202
وَالدَّلِيلِ عَلَى أَنَّ الْأَمْرَ بِذَلِكَ أَمْرُ اسْتِحْبَابٍ لَا أَمْرُ إِيجَابٍ؛ إِذْ صَلَاةُ النَّوَافِلِ فِي الْبُيُوتِ أَفْضَلُ مِنَ النَّوَافِلِ فِي الْمَسَاجِدِ .
And the explanation of the evidence that this command of the Prophet (peace be upon him) was for recommendation, not obligation, because offering voluntary prayers at home is better than offering them in the mosques.
