كتاب الإيمان
1  
The Book of Faith
(68)
Chapter: Being kind to one for whose faith there is concern because it is weak; Prohibition of attributing Faith to someone without definitive evidence
(68)
باب تَأَلُّفِ قَلْبِ مَنْ يَخَافُ عَلَى إِيمَانِهِ لِضَعْفِهِ وَالنَّهْيِ عَنِ الْقَطْعِ بِالإِيمَانِ مِنْ غَيْرِ دَلِيلٍ قَاطِعٍ
Sahih Muslim 150 b

It is narrated on the authority of Sa'd that the Messenger of Allah (ﷺ) bestowed upon a group of persons (things), and Sa'd was sitting amongst them. Sa'd said:

The Messenger of Allah (ﷺ) ignored some of them. And he who was ignored seemed to be more deserving in my eyes (as compared with others). I (Sa'd) said: Messenger of Allah I why is it that you did not give to such and such (man)? Verily I see him a believer. Upon this the Messenger of Allah (ﷺ) observed: Or a Muslim? I kept quiet for some time but I was again impelled (to express) what I knew about him. I said: Messenger of Allah why is it that you did not give it to such and such? Verily, by Allah, see him a believer. Upon this the Messenger of Allah (ﷺ) remarked: (Nay, not a believer) but a Muslim. He (Sa'd) said: I again kept quite for some time but what I knew about him again impelled me (to express my opinion) and I said: Why is it that you did not give (the share) to so and so: By Allah, verily I see him a believer. The Messenger of Allah (ﷺ) remarked; (Nay, not so) but a Muslim. Verily (at times) I give (a share) to a certain man apprehending that he may not be thrown prostrate in the Fire, whereas the other man (who is not given) is dearer to me (as compared with him).
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا ابْنُ أَخِي ابْنِ شِهَابٍ، عَنْ عَمِّهِ، قَالَ أَخْبَرَنِي عَامِرُ بْنُ سَعْدِ بْنِ أَبِي وَقَّاصٍ، عَنْ أَبِيهِ، سَعْدٍ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَعْطَى رَهْطًا وَسَعْدٌ جَالِسٌ فِيهِمْ قَالَ سَعْدٌ فَتَرَكَ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْهُمْ مَنْ لَمْ يُعْطِهِ وَهُوَ أَعْجَبُهُمْ إِلَىَّ فَقُلْتُ يَا رَسُولَ اللَّهِ مَا لَكَ عَنْ فُلاَنٍ فَوَاللَّهِ إِنِّي لأَرَاهُ مُؤْمِنًا ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَوْ مُسْلِمًا ‏"‏ ‏.‏ قَالَ فَسَكَتُّ قَلِيلاً ثُمَّ غَلَبَنِي مَا أَعْلَمُ مِنْهُ فَقُلْتُ يَا رَسُولَ اللَّهِ مَا لَكَ عَنْ فُلاَنٍ فَوَاللَّهِ إِنِّي لأَرَاهُ مُؤْمِنًا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَوْ مُسْلِمًا ‏"‏ ‏.‏ قَالَ فَسَكَتُّ قَلِيلاً ثُمَّ غَلَبَنِي مَا عَلِمْتُ مِنْهُ فَقُلْتُ يَا رَسُولَ اللَّهِ مَا لَكَ عَنْ فُلاَنٍ فَوَاللَّهِ إِنِّي لأَرَاهُ مُؤْمِنًا ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَوْ مُسْلِمًا ‏.‏ إِنِّي لأُعْطِي الرَّجُلَ وَغَيْرُهُ أَحَبُّ إِلَىَّ مِنْهُ خَشْيَةَ أَنْ يُكَبَّ فِي النَّارِ عَلَى وَجْهِهِ ‏"‏ ‏.‏
Reference : Sahih Muslim 150b
In-book reference : Book 1, Hadith 286
USC-MSA web (English) reference : Book 1, Hadith 277
  (deprecated numbering scheme)