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 d

The same hadith has been narrated on the authority of Muhammad b Sa'd and these words (are also there):

The Messenger of Allah (ﷺ) gave a stroke on my neck or between my two shoulders and said: Sa'd, do you fight with me simply because I gave (a share) to a man?
وَحَدَّثَنَا الْحَسَنُ الْحُلْوَانِيُّ، حَدَّثَنَا يَعْقُوبُ، حَدَّثَنَا أَبِي، عَنْ صَالِحٍ، عَنْ إِسْمَاعِيلَ بْنِ مُحَمَّدٍ، قَالَ سَمِعْتُ مُحَمَّدَ بْنَ سَعْدٍ، يُحَدِّثُ هَذَا فَقَالَ فِي حَدِيثِهِ فَضَرَبَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِيَدِهِ بَيْنَ عُنُقِي وَكَتِفِي ثُمَّ قَالَ ‏ "‏ أَقِتَالاً أَىْ سَعْدُ إِنِّي لأُعْطِي الرَّجُلَ ‏"‏ ‏.‏
Reference : Sahih Muslim 150d
In-book reference : Book 1, Hadith 288
USC-MSA web (English) reference : Book 1, Hadith 279
  (deprecated numbering scheme)