Ven. Ananda was the Buddha's attendant and literally very close to him. In MN121, the Buddha teaches:
MN121:3.3: ‘Ānanda, these days I usually practice the meditation on emptiness.’
MN121 is a beautiful sutta and very inspiring. MN121 is the Cūḷasuññatasutta, the Shorter Discourse on Emptiness.
MN121 is not secret or hidden, but it can be confusing given that the Buddha teaches about and distinguishes between space, nothingness and emptiness.
MN121:6.7: There is only this that is not emptiness, namely the oneness dependent on the perception of the dimension of infinite space.’
MN121:8.7: There is only this that is not emptiness, namely the oneness dependent on the perception of the dimension of nothingness.’
Yet this sutta can be read by all who seek peace. MN121 is not only an inspiring read but is also an actual instruction for meditation. And as an instruction to meditation, it refers to many concepts taught in other suttas.
In general, the more suttas we read, the better informed we are about which practice will provide a particular benefit for a particular person or situation.