Narrated `Abdullah:

While I was in the company of the Prophet (ﷺ) on a farm and he was reclining on a palm leave stalk, some Jews passed by. Some of them said to the others. "Ask him (the Prophet (ﷺ) about the spirit." Some of them said, "What urges you to ask him about it" Others said, "(Don't) lest he should give you a reply which you dislike." But they said, "Ask him." So they asked him about the Spirit. The Prophet (ﷺ) kept quiet and did not give them any answer. I knew that he was being divinely inspired so I stayed at my place. When the divine inspiration had been revealed, the Prophet (ﷺ) said. "They ask you (O, Muhammad) concerning the Spirit, Say: "The spirit," its knowledge is with my Lord; and of knowledge you (mankind) have been given only a Little." (17.85)

حَدَّثَنَا عُمَرُ بْنُ حَفْصِ بْنِ غِيَاثٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا الأَعْمَشُ، قَالَ حَدَّثَنِي إِبْرَاهِيمُ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ بَيْنَا أَنَا مَعَ النَّبِيِّ صلى الله عليه وسلم فِي حَرْثٍ وَهْوَ مُتَّكِئٌ عَلَى عَسِيبٍ إِذْ مَرَّ الْيَهُودُ، فَقَالَ بَعْضُهُمْ لِبَعْضٍ سَلُوهُ عَنِ الرُّوحِ، فَقَالَ مَا رَابَكُمْ إِلَيْهِ، وَقَالَ بَعْضُهُمْ لاَ يَسْتَقْبِلُكُمْ بِشَىْءٍ تَكْرَهُونَهُ فَقَالُوا سَلُوهُ فَسَأَلُوهُ عَنِ الرُّوحِ فَأَمْسَكَ النَّبِيُّ صلى الله عليه وسلم فَلَمْ يَرُدَّ عَلَيْهِمْ شَيْئًا، فَعَلِمْتُ أَنَّهُ يُوحَى إِلَيْهِ، فَقُمْتُ مَقَامِي، فَلَمَّا نَزَلَ الْوَحْىُ قَالَ ‏{‏وَيَسْأَلُونَكَ عَنِ الرُّوحِ قُلِ الرُّوحُ مِنْ أَمْرِ رَبِّي وَمَا أُوتِيتُمْ مِنَ الْعِلْمِ إِلاَّ قَلِيلاً‏}‏‏.‏
Reference : Sahih al-Bukhari 4721
In-book reference : Book 65, Hadith 243
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 245
  (deprecated numbering scheme)