Al-Kāfi - Volume 1 > The people of Dhikr (memory and knowledge) that Allah has commanded the creatures to ask for their questions are the Imams (a.s.)
Hadith #3

3ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ عَنْ مُعَلَّى بْنِ مُحَمَّدٍ عَنِ الْوَشَّاءِ قَالَ سَأَلْتُ الرِّضَا (عَلَيْهِ السَّلام) فَقُلْتُ لَهُ جُعِلْتُ فِدَاكَ فَسْئَلُوا أَهْلَ الذِّكْرِ إِنْ كُنْتُمْ لا تَعْلَمُونَ فَقَالَ نَحْنُ أَهْلُ الذِّكْرِ وَنَحْنُ الْمَسْئُولُونَ قُلْتُ فَأَنْتُمُ الْمَسْئُولُونَ وَنَحْنُ السَّائِلُونَ قَالَ نَعَمْ قُلْتُ حَقّاً عَلَيْنَا أَنْ نَسْأَلَكُمْ قَالَ نَعَمْ قُلْتُ حَقّاً عَلَيْكُمْ أَنْ تُجِيبُونَا قَالَ لا ذَاكَ إِلَيْنَا إِنْ شِئْنَا فَعَلْنَا وَإِنْ شِئْنَا لَمْ نَفْعَلْ أَ مَا تَسْمَعُ قَوْلَ الله تَبَارَكَ وَتَعَالَى هذا عَطاؤُنا فَامْنُنْ أَوْ أَمْسِكْ بِغَيْرِ حِسابٍ.

3. Al-Husayn ibn Muhammad has narrated from Mu‘alla ibn Muhammad from al-Washsha’ who has said that said to Imam al-Rida (a.s.), “May Allah take my soul in service for your cause, what is the meaning of the words of Allah, “Ask the people of Dhikr if you do not know.”? (16:43, 21: 7 Holy Quran) The Imams (a.s.) said, “ ‘Dhikr’ is Prophet Muhammad (s.a.) and we are his family (people) about whom questions will be asked.” I further asked about, “Are you the ones to be questioned and we will be the one to question?” The Imams (a.s.) said yes, that is true.” I then asked, “Will it a right on us to ask you?” The Imams (a.s.) said, “Yes, it is so.” I then asked, “Will it be a right on you to answer us?” The Imams (a.s.) said, “No, will decide. We may or may not answer. Have you not heard the words of Allah, the Most Holy, the Most High, that say, “This is a gift from us. You may (give to others and) oblige or keep without being held accountable.”(38:39)