Anas b. Malik reported Allah's Apostle (ﷺ) as saying:

Some persons from amongst my associates would turn to my Cistern; when I would see them and they would be presented to me, they would be detained in the way while coming to me. I would say: My Lord, they are my companions, they are my companions, and it would be said to me: You don't know what innovations they made after you.
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا عَفَّانُ بْنُ مُسْلِمٍ الصَّفَّارُ، حَدَّثَنَا وُهَيْبٌ، قَالَ سَمِعْتُ عَبْدَ الْعَزِيزِ بْنَ صُهَيْبٍ، يُحَدِّثُ قَالَ حَدَّثَنَا أَنَسُ بْنُ مَالِكٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ لَيَرِدَنَّ عَلَىَّ الْحَوْضَ رِجَالٌ مِمَّنْ صَاحَبَنِي حَتَّى إِذَا رَأَيْتُهُمْ وَرُفِعُوا إِلَىَّ اخْتُلِجُوا دُونِي فَلأَقُولَنَّ أَىْ رَبِّ أُصَيْحَابِي أُصَيْحَابِي ‏.‏ فَلَيُقَالَنَّ لِي إِنَّكَ لاَ تَدْرِي مَا أَحْدَثُوا بَعْدَكَ ‏"‏ ‏.‏
Reference : Sahih Muslim 2304 a
In-book reference : Book 43, Hadith 56
USC-MSA web (English) reference : Book 30, Hadith 5706
  (deprecated numbering scheme)