Mantra 9

अयं वाव स योऽयमन्तर्हृदय आकाशस्तदेतत्पूर्णमप्रवर्ति पूर्णमप्रवर्तिनींश्रियं लभते य एवं वेद ॥ ३.१२.९ ॥

In plain words

is the space that is within the heart. It is full and unmoving. He who knows this obtains full and unmoving prosperity.