Sahih Muslim 392
The Book of Faith
حَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ أَخْبَرَنِي نَافِعٌ، مَوْلَى أَبِي قَتَادَةَ الأَنْصَارِيِّ أَنَّ أَبَا هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " كَيْفَ أَنْتُمْ إِذَا نَزَلَ ابْنُ مَرْيَمَ فِيكُمْ وَإِمَامُكُمْ مِنْكُمْ " .
It is narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) observed:What will be your state when the son of Mary descends amongst you and there will be an Imam amongst you?
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.