The source is a letter to Lou Andreas-Salomé dated 13 January 1923. The context of the quote is as follows (emphasis mine):
Indessen: ich kümmere mich nicht allzuviel um diese, die mittleren Organe angreifenden Schwankungen; höchstens, daß ich meine Energie verwende, die vom Geist over vom Gemüt ausgehenden, intensiven Vibrationen für die Essensstunde ebenso "abzustellen", wie mir das ja dem Schlaf gegenüber meistens gelingt. Dieser große Gott: der Schlaf; ich opfere ihm ohne jeden Zeit-Geiz — was kümmert ihn Zeit! — zehn Stunden, elf, ja zwölf, wenn er sie annehmen mag in seiner erhabenen mild-schweigenden Art! Nur leider gelingt es mir jetzt selten, früh schlafen zu gehen; abends ist meine Lesezeit.
In the English translation by Jane Bannard Greene and M. D. Herter Norton (Letters of Rainer Maria Rilke, volume 2, page 320) this is:
Meanwhile: I am not worrying too much about these fluctuations attacking the central organs; at most that I should use my energy to "turn off" for the meal hour the intensive vibrations emanating from mind or mood just as I mostly succeed in doing with regard to sleep. That great god: Sleep; I sacrifice to him without any time-avarice—what does time matter to him!—ten hours, eleven, even twelve, if he wants to accept them in his lofty, mildly-silent way! Only unfortunately I seldom manage now to go to bed early; evening is my reading time.
This is the same letter that mentions Proust in a passage about not politically active people whose influence works in a very different way.
The letter is included in Mitten im Lesen schreib ich dir (published by Suhrkamp), from which you screenshotted the table of contents in the question Where else did Rilke mention John on Patmos?. See pages 194–197.
