Tadbhutasya tu natadbhavo jaiminerapi niyamatadrupabhavebhyah III.4.40 (465)

But for one who has become that (i.e. entered the highest Ashrama, i.e., Sannyasa) there is no reverting (to the preceding ones) on account of restrictions prohibiting such reversion or descending to a lower order. Jaimini also (is of this opinion).

Tadbhutasya: of one who has become that, for one who has attained that (highest Ashrama); Tu: but; Na: no; Atadbhavah: lapse from that stage, falling away from that; Jaimineh: according to Jaimini, of Jaimini (is this opinion); Api: also, even; Niyamatadrupabhavebhyah: on account of the restrictions prohibiting such reversion. (Niyamat: because of the strict rule; Atadrupdbhavebhyah: because there is no statement permitting it, and because it is against custom; Abhavebhyah: because of the absence of that.)

The question whether one who has taken Sannyasa can go back to the previous Ashrama is now considered.

The present Sutra declares that he cannot go back to the previous Ashrama. This is the opinion of Jaimini also.

There are no words in the Sruti allowing such a descent. The Sruti expressly forbids it, "He is to go to the forest, he is not to return from there".

It is also against approved custom or usage.

The Upanishad declares "Having been dismissed by the teacher he is to follow one of the four Ashramas according to rule, up to release from the body" (Chh. Up. II.23.1). There are texts which teach of the ascent to higher Ashramas. "Having completed the Brahmacharya state he is to become a householder. He may wander forth from the Brahmacharya state," but there are no texts which treat of the descent to lower Ashramas.

Dharma is what is enjoined for each and not what each is capable of doing.

Scripture declares, "Once returning to the forest, one should never return to household life." A Sannyasi should not stir up the household fire again after having once renounced it."

Therefore, one cannot go back from Sannyasa.