Imam Ahmad : 1537

حَدَّثَنَا عُثْمَانُ بْنُ عُمَرَ حَدَّثَنَا أُسَامَةُ عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ ابْنِ لَبِيبَةَ أَنَّ سَعْدَ بْنَ مَالِكٍ قَالَ سَمِعْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ خَيْرُ الذِّكْرِ الْخَفِيُّ وَخَيْرُ الرِّزْقِ مَا يَكْفِي

Narrated to us ['Uthman bin Umar] has told us [Usamah] from [Muhammad bin Abdurrahman bin Labibah] that [Sa'd bin Malik] said : I heard the Prophet peace be upon him say: "The best dhikr is that which is hidden and the best rizqi is that which is merely sufficient.